Author's Note: Again, rough draft. Will improve later. Just slapping this crap down as it comes to me.
The Codex of Lore
Crafted by a particularly nasty Grey Hag who had a propensity for deceit and a love for inspiring hubris. Anyone who makes a pertinent Planar Knowledge check (DC: 25) or casts an Identify spell will recognize the massive 3 foot wide (and 1 foot deep) flesh-bound tome with its grinning hordling face (and bronze supports) as the fiendish Codex of Lore, within which all written knowledge is contained. One needs only to ask the book for what they wish to read, then open it to find it reproduced. Basically, free castings of Legend Lore along with any spell the reader wishes to scribe or memorize.
Except it doesn't do this at all; if the roller manages to roll a 40 or higher, they're aware that this is actually an immensely clever, self-perpetuating lie promoted by the book itself. All information supplied by the book does nothing but tell the reader precisely what it is they want to hear--all legends are made up by the book, designed to convince the reader of their own personal infallibility. In addition, any spell that the reader uses this book to scribe or memorize is purely illusionary--a fireball spell memorized by the reader will produce an illusionary 'fireball' that only the caster can see.
Every use of this book for any one task requires a Will save versus a DC of 20. Failure causes an ability drain of a single Wisdom point; the victim is usually unaware of this. The reader becomes more and more obsessed with the book, until all their Wisdom is drained--at which point they fall into a coma and usually die of starvation (often, when found, the book is surrounded by ancient corpses; sometimes, one corpse may be bent over the book, still trying to read it).
I like the idea but have a single criticism: Right now its just a "trap" - one of those cursed items that has no practical use and lots of drawbacks, rather than the nicer (in my opinion) cursed objects that have enough useful properties to make them tempting to use. A spellbook fuelled by your own self-deception and pride is such a nice idea that I think it deserves to be more 'playable' - I think the example I'm looking for here is Stormbringer from Moorcocks Elric stories: a cursed artefact that's more of a bane than a help in the long run but just too useful to discard (although ideally something a little less apocolytically powerful!)