Books kinda sort block X-rays, but not fully. They look like plastic explosive. One of my college roommates was studying for two AP tests when selecting colleges, and he flew with college-level physics and chemistry textbooks. They caused mild alarm until the bag was unzipped. (Yes, this was before Nine! Eleven!)

Several years ago before the release of the Kindle, my mother was reprimanded by a TSO for carrying too many books. When she asked what she could do to pack her carry-on better to avoid secondary searches, the TSO told her, "Don't read."