What are your favourite obscure/less known books?

I noticed book threads mostly revolve around the usual suspects. Just like most other book lists outside of HN.

The idea behind this question is "If you only read the books that everyone else is reading, you can only think what everyone else is thinking."

(... which ironically comes from an author that everyone reads)

Let's see if we can compile some interesting list of good/useful books and novels that are hardly found in the mainstream/popular lists and expand our bookshelves as well as horizons.

Please add couple of sentences about the book too.