A couple people beat me to it, but Cornwell's Grail Quest trilogy (The Archer's Tale, Vagabond, and Heretic- all available in paperback currently) are excellent reads. Just finished Heretic, actually. I haven't read his Arthur series yet, though I plan to. For historic fiction, Cornwell just can't be beat.
(As an aside, for a very interesting piece of fiction, pick up Cornwell's Stonehenge- the story it tells isn't all that innovative and creative compared to some of his other work, but put in the context of what he is doing- essentially telling a possible origin of the creation of Stonehenge- it is a marvel.)
Also, as others have noted, the Grail pops up frequently in various of Moorcock's works. Seems to me there's another "modern" Grail quest that I enjoyed quite a bit that I can't recall offhand... hmm.