You are probably already familiar with the classic "Secret Book Compartment" project. On Instructables alone, you can find over 20 different variations of this project. The basic concept is simple. You hollow out a large book and you use the cutout space to hide stuff. Unfortunately, most versions of this project have one major problem. You can only hide objects that are smaller than your book. However it is possible to hide larger objects by combining multiple books into one large multi-book container.
Step 1: Select a Set of Books
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.
5. A good source for cheap books is community book sales. Many public libraries will have a large book sale
once a year. On the last day of these sales the remaining books are typically sold at a huge discount. You can often get a whole bag full of books for just a few dollars.
6. Get more books than you think that you will need. You must have one unaltered book on each side of your compartment, but I recommend having two or three so that if someone is messing with books on the shelf and a few fall over, it doesn't reveal your hidden compartment.