Phantom Unsafe Passenger Reading

I have so many library books on my front passenger seat that when I added the weight of my work bag to the pile it triggered my car's seatbelt sign sensor and the dashboard light began flashing warning me of an unbelted passenger.

Books on the seat:
The Bishop's Heir (read), The King's Justice (in bag), The Quest for Saint Camber, King Kelson's Bride by Katherine Kurtz
Dark of the Moon by John Sandford
Lean Mean Thirteen by Janet Evanovich
Kitty and the Midnight Hour by Carrie Vaughn