When I was a kid, spycraft was unbearably cool. One of the things I always wanted to make was a hollowed out book to keep secret stuff in.
One of the most important tasks is choosing the correct book.
The book must be large enough to contain whatever you want to hide. Also, it must be a book which would not look out of place on your bookshelves, but preferable be a bit dull, so people won't pick it up.
Oh, and always go for a hard-back.