What book or set of non-rpg books do you find you use between sessions for adventure or design ideas and reference for the next session more than any other?
Specifically, I mean book. As in "made of paper." Not looking for answers like Google or Wikipedia for purposes of this thread. Regardless of if you have it on a PC in pdf format, to list it here as a resource it must be in paper format, in your possession, not a rpg book, and reached for by you more often than any other book.
By set of books I don't mean Encyclopedia Britannica---a set of books to qualify for this would be one book/story broken up into several volumes. Like the Leiber Lankhmar series, or a 3 volume set of The Complete Works of Shakespeare for example.