What’s the strangest thing you ever found in a book?
I once took a critical thinking course and bought the textbook 2nd hand from the college bookstore.  A week or two in, I noticed half of a sentence written in the margin.  As the professor started teaching the topic from that page, he rhetorically asked a question, did not get an answer, and then answered himself with the sentence from the book.  I filliped ahead and found that the entire book was annotated with all of his answers, anecdotes, and various other helpful notes.  There was even a table that accurately listed his wardrobe choices!  The notes were in several different handwritings, and the book had been resold over a dozen times, so that professor must have been teaching the same class the exact same way for a decade or more.  I quickly became a star pupil as I always had an answer ready.  I added a few notes along the way and then sold it back to the bookstore at the end of the year.  I really wanted to keep it for posterity, but It just seemed wrong to take it out of circulation.