[b]The Codex of the Infinite Planes[/b] is a tome (technically a libram) of great power, known for causing madness and great catastrophes in its wake, but full of the secrets of the planes themselves. It can open a portal to any Plane, Demiplane, or prime+material world at any location, but it has no index or table of contents. Some of its users are thought to have become fiends or gods. It is somehow sentient, and has a nasty habit of enslaving wizards and forcing them to write parts of the ever-expanding libram in their dreams. Over time,
book drains the character's knowledge and life force until there is no
trace of the character (see Dragon #203, pg 74 for an example of this
The Codex, also called Yagrax's Tome, caused the sinking of the Isles of Woe. The wizard Tzunk (or Tzoonk) used the Codex in a mad attempt at conquering the City of Brass, but was brought down by the Efreeti army. It will mess you up.
Sources: "The Plane Truth" in Dragon #203, #204, and #205 [link to PDF]; Epic Level Handbook; 2e Book of Artifacts; 1st edition DMG; Eldritch Wizardry; Encyclopedia Magica, volume 2 (under "Libram"); Tales of the Lamp; d20 SRD; The Mimir