Bernard Cornwell writes a lot of excellent historical fiction. I've read his medieval stories--one about a quest for the Holy Grail in 1300ish England and France, and the other his version of King Arthur. (Just saw Griff recommended these, as well. Heed our advice.
Dorothy Dunnett has written a few good series which mostly take place in Scotland in the 1500s.
As far as nonfiction goes, Bernard Lewis has written a lot about the Middle East. His books are well written and lucidly realized.
Alan Watts is one authority on Zen Buddhism and his books are also excellent.