Reddit, what's your favorite obscure or lesser-known book that you hardly hear recommended?

Many of us, myself included, have made our way through many or most of the classics and popular literature, what books might we have missed along the way?

EDIT: You all have been AWESOME. Thank you so much for the recommendations and any future ones added on. I did my best to make a thorough Goodreads list with all the selections presented and recommended to me, that I haven't read (which was only a few here and there).