Underrated literature

I'm currently searching for relatively unknown, well-written books. I'd prefer a classic or a book from the 20th century, but it's not a necessity. I'm also interested in reading historical literature

I want to expand my horizons, because I noticed the vast majority of the books I've read were fantasy or non-fiction. I've only read the most famous classics, such as Lord of the Flies, Hamlet, Alice in Wonderland and so on

Along with a recommendation, I'd like to hear the "why": why is it well-written, why is it underrated & why do you think it's worth reading? I don't mind spoilers, so you can explain in detail if you'd like