I once had a copy of 1984 in my checked baggage returning home to the USA and when I was unpacking the bag at home, the book had a notice inside the book that my bags had been inspected by TSA...
I reckon they were just checking for money or hidden compartments. Sending books abroad packed with money is extremely common from the US, though not sure how frequently people do that with onboard luggage.
Only tangential but when I was an undergraduate studying philosophy I had Bertrand Russell's Why I Am Not A Christian in my carry-on, and the TSA saw that and had a field day.
I hope the TSA agent got a chuckle as he put the notice in the book. I would assume his choice was deliberate.