And then there are the ones who like to correct the grammar, or point out inaccuracies in the plot!
I borrowed several detective books set in a local town from the library. Occasionally I found tiny neat pencil notes in the margin setting the author straight. Typical examples:
"The local newspaper in this town is called the Argus, not the Crier. This is wrong."
"There are no trains running direct from Greenville to Redbridge and therefore this plot has holes in it."
"There are no traffic lights at this junction. Ridiculous!"
Clearly the concept of fiction eluded the note-maker. The books weren't brilliant, but I thoroughly enjoyed the increasingly irate corrections.