Is it that the reflection of the already-on LED (or maybe external light at the right angle) is hitting the sensor used for NOT and then creating a feedback loop to keep the light on when the switch isn't pressed? This happened a few times, and either changing the ambient light or holding the book tightly shut will resolve the issue on my copy.

edit: because the author is here, thanks for the book: an interesting idea and well-executed. Daughter doesn't mind the front page blue LED. I am more amused than annoyed: even a two-transistor circuit needs debugging sometimes.