Keith Roberts - Pavane - because it's beautiful, because it's a ground-breaking example of alternative reality fiction, because I will never stop re-reading it, because of its subtle Celtic mythology.
Diana Wynne Jones - The Homeward Bounders. Quite possibly her most underrated book. Beautiful use of mythology and human tragedy
Tim Powers - The Anubis Gates. Because I love the time travel aspect of it, because he really knows his fencing and because he interweaves poetry and Egyptian mythology.
L. Sprague de Camp and Fletcher Pratt - The Roaring Trumpet. Yes, Norse mythology. It's also very witty and clever, and I just love the cockroach racing scene in the fire giants' jail between Heimdall and the protagonist. Cracks me up every time.
Terry Pratchett - Night Watch. The best Vimes book of the lot, although all the Vimes ones are stunning.