Dean Esmay is asking for recommendations on good books which aren't widely known. He mentions Buddy Holly is Alive and Well on Ganymede by Bradley Denton and Ariel by Stephen R. Boyett. Readers have offered their own opinions, including a bunch which I wouldn't consider unknown, by authors such as Patricia McKillip, Ursula K. LeGuin, Poul Anderson, and Stephen R. Donaldson, who certainly aren't unknowns (to SF readers, anyway). Admittedly, it could just be that I don't consider anything I've read unknown.
Sadly, no one's mentioned Fire yet.
Let's see, is there anything I'd add to the list? Frankly, most of the books in my library are popular books from popular writers. How about Jane Jensen's The Beast Within? Jane Jensen designs computer games, specifically the Gabriel Knight series, which are fabulous games with intricate, engrossing plotlines. Her first attempt to convert a game into a book, Sins of the Fathers, didn't turn out too well, but her second book was very good. I'm not sure she ever tried to turn the third Gabriel Knight game (Blood of the Sacred, Blood of the Damned) into a book, but it was the weakest game in the series (not to mention host to some religious elements I found disturbing). I'd also recommend John Ringo, whom I think is pretty well-known, so I'm probably not recommending an unread author. He's written some intriguing Sci-Fi series, including A Hymn Before Battle and There will be Dragons, and he makes it clear in his books that he's a Sluggy Freelance fan.
New Post: I note that Doc Rampage has made some suggestions above.