This is one of the hardest topic for me. I almost want to skip this but that would be cheating. I can't think of any book that can be called as underrated. Some are obscure, yes, but it is deserving of being obscure. I am not involved enough in the book community to know if a book is well-received or not.
Then I suddenly thought of Georgette Heyer, A Civil Contract. An author and the book that I kinda love but I rarely heard people gushing/talking about.
The synopsis are:
The romantic plot centers on a viscount who reluctantly enters a marriage of convenience with a wealthy commoner's daughter.
While I used to love Georgette Heyer books due to the predictable romantic formula rich guy meet not so rich girl, not exactly love at first sight but fall in love madly. This story is more realistic. The guy marry a rich girl to save his family. The girl is not a great beauty. In fact she is very plain and in the chubby side.
It deals with what to do when our dream falls to pieces and we are left to deal with harsh reality. (While granted this harsh reality is not exactly dirt poor/traumatic experience type of harshness) . I love the book I love all the characters, even the flighty, vapid one. The thing about Georgette Heyer is that even her most weakest character are redeemable.
While I love this book, why I classed this 'underrated'? Like I said, I gathered Heyer was/is rather popular because she got her ardent followers. However to my recollections her books are rarely mentioned or gushed about. So I place her in the underrated, because while it is not exactly Austen with her sharp wit, Heyer have her moments and this is one of her stronger book which I thoroughly enjoyed reading several times over.