3 books, but you can find it in a hardcover compilation. Has nothing to do with the tv series, or Buffy the Vampire slayer. But there are vampyres. Consider this a "girl's quest to save a fallen angel" kind of story, only with a futuristic setting, and a good use of landscaping on the Earth's moon.
The Cycle of Fire by Janny Wurts
3 books (Stormwarden, Keeper of the Keys, and Shadowfane). I really liked this series the first time I accidentally picked up the second book (*blush*), and continue to like it even today. Another fantasy series with a sci-fi background.
The Farseer Trilogy, The Liveship Traders Trilogy, and The Tawny Man(?) Trilogy by Robin Hobb
I highly recommend this triple set of trilogies. All of them are tied together, but each trilogy can stand alone. You only get the whole picture when you read 'em all, though.
If you like cliched approaches to grand quests, and heroes who never have problems they can't live through, you won't like it. I spent most of the first two books in the Farseer Trilogy convinced that Ms. Hobb hated her main character. But it was FitzChivalry's sheer tenacity at wanting to live despite his problems, and in just the way he lives that kept me reading until the last book hit the stand. And I am so very glad I did.
Oath of Swords, by David Weber
(note: I'm pulling most of these titles and authors out of my head, so if I'm wrong, my apologies)
Funny book. Just, dang good fun!
Part of a "trilogy" of sorts, but I'm tempted to let it slide until he finishes it. This book and the subsequent "The War God's Own" are the best, so far.