I should have known. When I came back to China a few hours ago, returning from a five-week trip to Pakistan and Nepal, I was only concerned about one thing.

I had an extra bag with two small Afghan carpets that I had bought in Islamabad.

Would customs stop me and charge me duty?

My Air China flight took me first to Lhasa, where we went through customs and immigration, then on to Chengdu and Beijing.

Sure enough, the agents stopped me at Lhasa airport even before I could pick up my bags. But it wasn’t the rugs. Instead, it was the little cloth bag with five new books I had bought in a Kathmandu bookstore the night before.

One security agent signaled another one over, who knew some English, to peruse the books. They asked me to take off the plastic wrap around two of them. He opened each one and flipped through the pages. I thought maybe he believed I had sliced out a secret compartment in the middle of the books, which obviously I hadn’t done.

He took particular interest in one book: Buddha’s Warriors, by Mikel Dunham, a 2004 account of how the CIA helped turn peaceful monks into armed warriors to fight the Chinese invasion of Tibet. I haven’t read the book but best as I could tell it was a historical review of a brief and long-forgotten U.S. policy during the early Cold War era.

Here’s the problem: The book has photos, including of cadres during the Cultural Revolution belittling class enemies in mass rallies in Tibet. The agent studied the photos, and quickly looked at me. “This is false history,” he said.

Astonished that he could make such a quick determination, I said that the book was about a failed U.S. policy more than four decades old. He was not moved. He suggested that I could buy “true” histories of Tibet at the main market in Lhasa. Reminded that I wasn’t staying in Lhasa, he just shook his head and said the book was confiscated.

I’m sure it was for my own good.