Codex of the Infinite Planes
The Codex of the Infinite Planes is a Dungeons & Dragons artifact hailing from the White Box Supplements. It then went on to appear in the AD&D 1e DMG, AD&D 2e Book of Artifacts, D&D 3e's Epic Level Handbook, and D&D 4e's Mordenkainen's Magnificent Emporium. It also appears in Pathfinder's Artifacts and Legends.
Throughout its appearances, the Codex remains firmly defined; this massive book of inscrutable origins holds many powerful secrets relating to planar magic, secrets so powerful that even opening the book can be potentially deadly to a would-be user. Whilst its powers have fluctuated over the editions, the most common themes revolve around the ability to teleport users throughout the planes, enhancing their magical abilities, and access to various conjuration spells - though always at a steep and probably deadly price.
3e
Appearing in the Epic Level Handbook (and therefore in the OGL), in this edition the Codex is a nasty piece of work. Opening it up for the first time will hit you with Destruction (DC 30 for 10d6 damage: if you fail you turn to dust). If you manage to survive this you get to study its infinite pages for great knowledge and power... at a risk. After a day of study you can roll a DC 50 Spellcraft check to see if you learned something new: if you don't make the roll you get +1 to your next one. You can learn quite the list of powers, who once known you can use at will: astral projection, banishment, elemental swarm, gate, greater planar ally, greater planar binding, plane shift, and soul bind. Powers are learned at random. But the risk is always there: every day also requires a DC 30 Will save or you go insane (as per Insanity).
Actually using a power is risky as well: you need both a Concentration and a Spellcraft check with a DC 40 + double the spell's level (making it 50 at its lowest and 58 at its highest). If you fail, you roll a percentile dice to decide your fate: get hit by an Earthquake and Storm of Vengeance spell, summon 1d3+1 Balors, Pit Fiends or similarly powerful evil Outsiders to mess you up, have your soul put in a gem (as per Trap the Soul) and your body entomed somewhere (as per Imprisonment) both in a random place on the current plane, or utter a Wail of the Banshee and get hit by Destruction every turn for 10 turns.
All in all, if you find the Codex of the Infinite Planes it would be best not to use it.
Pathfinder
Because it appears in the SRD, Paizo decided to use it as well. It works more or less like it's 3e equivalent, except the "you done fucked up" chart is expanded to two dozen results of a varying number of DO NOT WANT, from having 444 Shinigami show up and try to murder people to being hit by Energy Drain or Reincarnate every day, grow to 65 foot in size and gain 5 feet every day, having everyone around you turn alignment, get hit by a series of Meteor Swarms, a zombie apocalypse stars and others.
And since the Codex is a book, this means that it's a legal target for a Tome Eater archetype Occultist to devour. The text is a bit murky as to if eating a book constitutes tearing out its pages, but if given the opportunity to do so you should totally do this if only to see your DM's face when you tell him you want to eat the Codex of the Infintie Planes.