I went to bed around 9:30 last night. Woke a few times before waking… - Queue
From the other side of the fence, I always enjoyed running into former students and chatting about what they were doing. What I did find kind of problematic were the few situations where a student clearly had a lot invested in the interaction (particularly young female students). So I'd suggest writing the dear a friendly but somewhat offhand note, on the order of "Hi: we're publishing your book, and I think that's kind of cool".