It's given stats in the 3.5 ELH, but they don't do it as much justice as the 2e Book of Artifacts, or a series of articles 'The Plane Truth' by David Cook. Cook's reinterpretation of the Codex had that the book (and pardon my retelling of this, I might be off slightly), whatever its origin, continually added to its seemingly infinite page count by reaching out to mortals and sending them dreams, visions of other places, and then compelling them to write these things in a version of the codex found in their dreamscape. When awake, those mortals would inevitably seek out the places from their dreams, enacting in the physical world the events and circumstances that the Codex had revealed to them.
And all the while, the Codex slowly devours them bit by bit, draining their essence like a wizard casting washes till he's reduced to a broken husk.
Sometimes though, a person touched by the Codex manages to send out dreams of themselves to another, filling some hapless soul with memories and visions of the first dreamer, the 'slave of the codex'. This second person begins to find their skin covered in tattoos, words telling about the nature of the codex's slave who seeks to free themselves. Ultimately, before the codex drains them dry, the first dreamer manages to usurp the body of the second person, forever wearing the body of another for the rest of their life, and leaving that victim to die in their place at the touch of the Codex.
As for the actual physical Codex itself, it was used in some capacity by the Archmage Tzunk, in his attempt to conquer the City of Brass. His swathe of destruction through the city is still visible today, though it has, in the innumerable years since, been paved over and rebuilt to some extent. As for Tzunk himself... well... he was ripped to shreds by the Efreet and never actually managed to take the City of Brass itself, seemingly abandoned by the Codex in his moment of need.
And that's the thing with the Codex. Every time it's power is invoked, there's a increasing chance that something absolutely horrible is called down upon the holder of the book.
It'll be some time, but eventually in my 2nd storyhour, I have a side plot that revolves around the Codex, one former 'slave of the codex', and the ultimate fate of Tzunk.