I recently read a sci-fi book from the library. A previous borrower had jotted notes/corrections/criticisms of the author's science in the margins, in the tiniest, neatest handwriting I've ever seen. None of the "problems" they pointed out detracted from the story, but I guess they really wanted to show off their personal theories of FTL travel to whoever borrowed the book next. It was hilarious, and I'll remember the book forever because of that.

That's really cool. In some sense, it's like a precursor to internet fora.