It seems like a decent number of off-topic questions are book requests/advice on which book is best for topic X. Someone should add to the FAQ section on "What not to ask" a line:
What is a good book about __________?
Admittedly, I'm not sure how many people read that section of the FAQ before asking a question, but it seems common enough that it should fit in the appropriate section of the FAQ.
This is similar to Can we add "recommendations" to the list of "What kinds of questions should I not ask here" in the FAQ? but when I read that I think of what IDE, or language, or design pattern is best, not a book recommendation.