Hollowing out a book is a classic way to hide things in plain sight, but the space is very limited. If you have larger-than-one-book objects you'd like to secure, check out this multi-book hidden storage idea.
As the video above shows, all you need to do is get books that are taller and wider than the object to hide and carve out a space for it. Optionally, glue the books together to make the set easier to remove.
Make offers a few handy tips: use a power saw if you have one, pick up books for cheap at a book sale, and get more books (hardcovers preferable) than you need.
As an alternative, you could just glue some book spines to a box for a secret bookcase box.
Multi-Book Secret Storage Compartment [Instructables]