I don't think I could pick a best book -- many on my list have already been mentioned, some several times, but here are a couple to add to the pile:

Scaramouche by Rafael Sabatini -- historical novel that bears little or no resemblance to the movie with Stewart Granger (they took the title and the gimmick or "McGuffin," but little else). Sabatini has a thick Victorian style, which turns many people off, but his historical research is impeccable and he's one helluva story-teller. Errol Flynn made a career out of play (badly) Sabatini heroes. Whacking good adventure!

The Sparrow by Mary Doria Russell -- Hard to classify: mainstream/science fiction/religious/philsophical. Good story, but much more than that. It raises some uncomfortable questions -- definitely thought-provoking. She wrote a sequel, Children of God, which is almost as good, but you need to read The Sparrow first.

Don Firth