Imperial Armour
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.