If you've ever wanted a secret compartment book for your study, making your own is actually pretty easy.
Photo by haven't the slightest.
To make your own secret compartment book, Hannah Margaret Allen at Food52 recommends you get a thick, old book you're willing to carve into, a box cutter or Stanley knife and some Mod Podge glue. Open up the book so you're about 1/8 through the pages, use a rule to trace where the compartment will be, then carefully cut into the pages with your blade.
You'll probably only be able to cut through a few pages at a time, so take your time and be cautious. After you've removed enough pages, cover the open page and compartment with Mod Podge to keep it all stuck together. Close the book with some was paper inside like a bookmark and rest something heavy on the book overnight.
If you can't find a thick enough book, you can make something similar using multiple books.