So here's what I did. I googled "underrated books" and looked at the lists of books lacking in ratings on goodreads.com, which isn't really the same thing I guess, but whatever?? And I spotted one that was actually in my Day 2 post.
Maurice, by E. M. Forster
Yo, not only is this book great, it's an important cultural document. A novel about a young man coming to terms with his sexuality in a difficult time period, written from real life experience. Forster doubted it would ever be published due to its themes. But it was, eventually, and not enough people know about it.
I guess I don't have much more to say about it, except I need to read it again. Here's a promotional photo from the totally sexy Merchant-Ivory film adaptation: