I suppose I should take a look at how many pages there generally are to the inch. The inside of the book has to be a minimum of 1" thick. I had a couple in mind that turned out to be too thin.
Alternatively, haunt a second-hand book store and buy up all the ancient hard-backs and use them.
Who’s going to look for a hip flask buried in one volume of a 1950’s set of Encyclopedia Brittanica??