I've got a copy of Chronicles of Nchuleft and when I'm trading it disappears from my Inventory list! I know that Quest items do this so you can't accidently sell them, but I don't have a quest concerning this book although I do believe there is only one copy in the game. Any ideas ?

Did you steal it by chance?

^^

Stolen items won't appear in your inventory while you're trading unless you're with a fence.

I believe, and you can take that for what it's worth, that regardless if you have a quest for an item, if it is attached to a quest, it will not let you sell it, because at some point you might get the quest for said item.