Here's a great Instructables for hiding a stash-box behind a wall of cut-away books. In some ways, it's a lot less fiddly than creating a single hollow book, though it does require you to use a scroll saw.
1. The height and depth of the books are the important dimensions of the book. The size of the cover is what limits storage space. Since you are using multiple books, the thickness of each one doesn't matter.
2. The books do not need to be the same size, but it is convenient if they are. Sets of reference books like encyclopedias work well because they are all the same and it is a reasonable excuse to have a bunch of large books all in one spot on the book shelf.
3. Hardback books hold their shape better than paperbacks do after being cut up.
4. The secret compartment only remains a secret as long as no one tries to read any of the books. So it helps if the book are relatively uninteresting while still looking like something that you would have on your shelf.