Oh this would be fun! Hidden passageways concealed by bookcases are a must for creepy castles, superhero mansions, and my yet-to-be-built, yet-to-be-designed, yet-to-be-funded, house.
In the video below, the switch is a Jules Verne book. I’d think you would want the book to be one no one would have interest in grabbing. Maybe one on actuary tables?