Imperial Armour
Imperial Armour is a series of books written by Forge World (the company) to promote their Warhammer 40,000 models. They include full-color drawings, technical data, rules (in some cases full army lists, recently given an official stamp of approval), and, in most cases, a narrative focusing on a campaign which features the models included.
As the name might suggest, there is a bias towards Imperial armies -- at least one of the Space Marines, Imperial Guard, Imperial Navy, Sisters of Battle, and/or Inquisition have been represented in every book, although Forge World does seem to be giving less-seen forces some time in the spotlight, like the Raven Guard, Salamanders, Elysian Drop Troops, and Death Korps of Krieg. However, each book features a non-imperial army foremost (read: on the cover), and releases specialized units, army lists, and Apocalypse love for the specific non-imperial faction being featured. Tau got most of their Apocalypse units from their release, and the Ork list has a whole bloody tactica. More often than not, the xenos even win! Oh who do we kidding - Imperium forces was fucked up in every book - they either outright loose or their victory is totally Pyrrhic.
This volume has no over-arching story, though it does offer brief insights into the Imperium, such as tank doctrine and Forge World operations, as well as coloring references and technical data. Most of the vehicles included are Leman Russ variants and artillery pieces that were absorbed into the next Imperial Guard codex (this book was written back in 3rd Edition), and the other vehicles (Baneblade variants, Imperial Navy flyers, and support vehicles) have all been included in later Imperial Armour volumes -- note that this is the only volume not sold by Forge World any more. The "Armoured Battlegroup" army list (featuring Leman Russ Battle Tanks as troop choices!) is introduced in this volume.
Volume Two - Space Marines and Forces of the Inquisition
Again, no big story, but lots of lore about the many vehicles and their variants used by the Space Marines and Inquisition (which includes the Grey Knights and Sisters of Battle -- rather, the Daemonhunters and Witch Hunters, as they were called at the time).
Volume Three - The Taros Campaign
The first Imperial Armour volume with a proper story and campaign, in which the Tau take center stage. The story goes that the desert world of Taros gets investigated by the Administratum, as its tithe and production estimates are out-of-date. The delegation finds that, while Taros is meeting its mineral quotas to the local Forge World, it seems to be mining much more without growing its stockpiles; they discover that the planetary governor has been cutting a deal with the Tau. They enlist the aid of the Avenging Sons Space Marine Chapter, and when that attack fails, the Imperial Guard are called in. They also fail to re-take the planet, and the Tau actually win!
The Tallarn Desert Raiders and Elysian Drop Troops receive rules in this volume, as well as lots of new Tau units (like the mighty Manta, and the Orca from the Fire Warrior video game). The Raptors also got a little screen time, with some handy illustrations of their color schemes.
This volume is received an update to make it more in line with the new 6th edition rules. The Elysian Drop Troops also got a new army list in this book, but it's not really that great...
Volume Four - The Anphelion Project
The Adeptus Mechanicus decide that it would be a good idea to keep a zoo of Tyranids on Beta Anphelion IV, in order to study them and learn how to fight them better. Contact is, naturally, lost, and so Inquisitor Solomon Lok of the Ordo Xenos gathers up some Red Scorpions and Detachment D-99 of the Elysian Drop Troops to investigate. They learn that the Tyranids adapted to escape containment, Jurassic Park IN SPACE ensues, and the Marines escape while leaving Lok and the Guard to die horribly. Needless to say, this doesn't earn them many brownie points with the Inquisition.
The Tyranids get most of the love in this book, with new biomorphs, Monstrous and Gargantuan Creatures, and big flyers, though the Red Scorpions and Elysian Drop Troops get a few more rules.
Volume Five - The Siege of Vraks - Part One
Vraks, an Armory World close to the Eye of Terror and a key point in the defense of Cadia, falls silent. It turns out that Cardinal Xaphan has taken over, deciding that the Decree Passive was for weaklings, and that he could rid the Imperium of heretics much more effectively by taking direct command. The Imperium disagrees (once they find out about this), and decides that the fastest way to bring Xaphan to heel is with the Death Korps of Krieg. Unfortunately, they weren't counting on Lord Arkos of the Alpha Legion showing up. Still, they manage to crack the first two defense lines, but it seems that Lord Arkos and Cardinal Xaphan aren't out of tricks yet...
The Death Korps of Krieg Siege Regiment gets army rules in this volume (for which we have a tactica), as do the Renegades and Heretics (basically Lost and the Damned for 4th Edition -- Traitor Guard plus Alpha Legion). Some new super-heavy vehicles, the Macharius Heavy Tank and the Gorgon Armored Assault Transport, as well as special siege equipment, also get rules.
Imperial Armour Apocalypse
After years of experimentation (including experimental rules in previous Imperial Armour volumes), Games Workshop releases the Apocalypse expansion for running massive Warhammer 40,000 games. Forge World releases this volume to update the rules on their super-heavies and flyers, as well as include some new formations.
Volume Six - The Siege of Vraks - Part Two
As the Kriegers wear down the final defense line, Arkos summons reinforcements. In particular, lots of Berserkers and Plague Marines arrive, and then the Chaos Titans and flyers come in to attack the Krieg lines from the rear. In response, Segmentum Command sends their own Titan legion and Imperial Navy support, as well as reinforcements for the Kriegers. The Engineers manage to mine and collapse a whole section of the fortress wall, and the Kriegers surge into the gap. Unfortunately, they are held there. The Red Scorpions are asked to help, and they agree, seeing as there are hundreds of Chaos Space Marines to be killed. Though few of the Marines committed survive the assault, they manage to clear the breach and leave it in Imperial control. With the citadel itself in view, the Kriegers prepare to mount their final assault, when Inquisitor Hector Rex shows up and assumes direct control -- it seems that the taint of Chaos is even worse than any had expected...
This volume contains rules for Death Korps of Krieg Engineers and associated mining equipment (like the Hades Breaching Drill), the Red Scorpions, and the "Servants of Slaughter" variant of the Renegades and Heretics army -- basically, Traitor Guard with Khorne stuff. New units include another Macharius variant and the Malcador tank, as well as some updates on rules for Chaos Space Marine vehicles.
Volume Seven - The Siege of Vraks - Part Three
Under Inquisitor Rex's command, the siege around the citadel is sealed tight, and the Kriegers tighten the noose. Unfortunately, the forces of Chaos are hard at work assembling a massive warp portal, and they unleash daemons onto Vraks. The Red Scorpions return, along with some Grey Knights summoned by Rex's urgent missives, and with their support, the citadel's defenses are broken, but not before An'ggrath the Unbound is summoned by the heretics. He gets cast back into the Warp by Inquisitor Rex, and then the Angels of Absolution (successors of the Dark Angels) arrive, to settle the score from Volume Five and take some prisoners. They destroy the leadership, and the Grey Knights clear out the deepest levels of the citadel and finally close the warp portal. In the end, all of the ammunition and stores on Vraks were expended in the siege or destroyed afterward due to Chaos taint, the Ecclesiarchy relics and assets on Vraks were used for Chaos rituals (like a few Sisters of Battle recovered from the dungeons -- they were alive, but given the Emperor's Mercy) or lost, and nothing was really gained. About the only positive outcome was that the Traitor Titans lost more than the Loyal Titans.
The Death Korps of Krieg get rules for an "Armoured Battlegroup" variant army (Leman Russ Battle Tanks as Troops!), and the Renegades and Heretics get a new "Servants of Decay" variant, as well as some new Daemons and Daemon Engines.
Imperial Armour Apocalypse II
More Apocalypse updates for 5th Edition.
Volume Eight - Raid on Kastorel-Novem
The Raven Guard and Elysian Drop Troops make a raid into Ork-controlled space to try and eliminate a particularly bright Mekboy. It doesn't end well -- after heavy losses, they are forced to withdraw, having failed to locate him (though production is temporarily disrupted).
The Elysian Drop Troops' army list gets updated (see our tactica) for fifth edition in this volume, and the Orks get a new heavily-mechanized list variant, the Dred Mob. This book is where most of the Apocalypse units for Orks now come from.
Volume Nine - The Badab War - Part One
Lugft Huron decides that he can run his Sector better than the Imperium can. The Imperium disagrees and sends in the Space Marines to put him in his place, starting what would later be called the Badab War. Huron calls in his favors with the other Maelstrom Warders to keep control of the sector, though the Loyalists close in around him.
This volume has rules for the "Tyrant's Legion," forces under Huron's command. They are essentially Imperial Guard with some Space Marine support (or Marines with meat-shields, depending on your perspective and play-style).
Volume Ten - The Badab War - Part Two
The Imperium tightens the noose around Huron, and as his heresies come to be known, his allies start to turn from him. Eventually, the remnants of his forces flee into the Maelstrom. The rest is history.
The centerpiece of this volume is the "Siege Assault Vanguard," a heavy assault Space Marine variant army, including the Caestus Assault Ram as Fast Attack and Ironclad and Siege Dreadnoughts as Troops.
Imperial Armour Apocalypse Second Edition
Enough time has passed that Forge World has to update the rules for old models and release rules for new ones. As a sign of Forge World becoming more "mainstream," this is the first volume to have a "stamp of approval" on its rules.
Volume Eleven - The Doom of Mymeara
Imperial Guard and Space Wolves versus Eldar. The Eldar of Alaitoc and Mymeara, with Corsair support, are looking for the armor of Irillyth, the Phoenix Lord who founded the Shadow Spectres Aspect Shrine on Mymeara (he went out to preemptively eliminate the titular Doom, but never returned). They sense his essence after his millennia-long absence on the ice-world of Betalis III, and fight their way into the manufactory complex to get him, under cover of a massive aerial assault. They successfully recover his armor and re-awaken his spirit. Though Eldar sustain heavy loses during retreat after reaching their goal, so this time its more tie then pure xenos victory as usual.
This volume introduces the Warhammer 40,000/6th Edition Tactics/Eldar Corsairs army list (lots of fast things and flyers, with a complete Tactica), as well as a new Path for Craftworld Eldar Aspect Warriors (the Shadow Spectres) and a new Wolf Lord character.
Imperial Armour Aeronautica
As 6th Edition now includes flyers, lots of Forge World models need to have rules updated (and lots of armies need anti-air support).
Volume One - Second Edition: Imperial Guard
With 6th Edition out the door and no Imperial Guard Codex expected anytime soon, Forge World is releasing an update to Volume One which includes updated rules for all of the tanks and super-heavy vehicles available to the Imperial Guard. The Armoured Battlegroup army list(Leman Russ tanks as troops!) gets an update as well.
Volume Twelve - The Fall of Orpheus
Minotaurs and Death Korps of Krieg vs. Necrons. The story, as far as we know, seems to be that the Orpheus Sector turns out to be home to several Tomb Worlds, and the Necrons who wake up there (the Maynarkh Dynasty) are not interested in sharing. As far as models and rules go, it looks like the Necrons are getting a new bomber (the "Night Shroud"), some kind of personal hover-cart thing (the "Tesseract Ark"), a new tomb stalker variant (the "tomb sentinel"), a walking mini-Pylon (the "Sentry Pylon"), and presumably updates for already released Forge World Necron models. The Death Korps get a new "Assault Brigade" army list and the Necrons get a new "Dark Harvest" army list, while the Space Marines get some new Minotaurs characters.