A much more sophisticated way to keep electronic watch over your secret book safe's contents, is to run the Haven app on a burner Android phone. The open source Haven software was designed by The Guardian Project, the Freedom of the Press Foundation, Edward Snowden, and Micah F. Lee to turn any Android phone into a physical intrusion sensor.

Haven uses sound, light, vibration, and image motion detection to sense environmental change, store local logs of all activity, and can be configured to send alerts to your personal phone over SMS or with the end-to-end encrypted Signal app.

The app was built to run on older phones, so you can run it on an old Android phone you've got sitting in a drawer somewhere, or pick up a cheap burner phone. Install and configure the app as shown in the Haven docs. Once you've got it set up, you can hide it, along with your top secret items, inside your hollow book safe!

Any snoop that comes along and finds your secret book will be photographed and recorded in the act, with an alert going to your personal phone immediately.

You could also modify your secret book with a small hole positioned in front of the phone's camera to create a hidden Haven device to keep watch over your whole room.