I've been thinking of doing something like this for my game.
I can get empty bound books pretty cheap. Hobby Lobby, Michaels and a couple other art shops sell them. To keep it affordable, I'd try something smaller, paperback sized. Large books cost more.
I'd want to have a prop book that had relevant clues or information in it. A journal would be good for this. A wizard's journal would probably be full of drawings, arcane spells, journal type entries, notes and observations on things.
I'd want the book to contain relevant clues to my game AND be populated with enough material that finding those clues is a challenge. So instead of saying the PCs find a wizard's journal, have them roll some dice, and tell them they find a note about the Demon Prince of Azur, I'd just hand them the book they found and let them have at it.
The tricky part in all this is the types of content needed. We'd need spell book content, some hand-drawn art, some general journal entries, and some clue entries that have bearing on the game. The trick is coming up with enough content to fill the book so it looks complete enough.
Take Journal entries for instance. Obviously, the last entry is the one before the writer went mad and disappeared. But before that, you need entries leading up to it, as well as completely mundane entries that show the general life of the writer.
If this were a PDF product, I'd see it being used in two formats, something that you'd print out and "bind" and use as a prop, or something you'd transcribe into a more authentic looking book. Either way, the content would have to be good, to make it a believable prop.
hmm, gets me thinking....
Janx