Chrestomanci is a character in at least 4 of Diana Wynne Jones's books. He
features as the title character (I think as "Christopher Chant") in the fourth
of the books to be published, but I consider it the least of the 4 and would
read it last, though it is chronologically the first. He usually appears as
something of a deus ex machina, though, in _Witch Week_ and the other one that I
can't recall the title.... In _Charmed Life_ he has a bigger part. That might
be the best one to read first, if so, not knowing his character might be good
for the suspense of the book.
Jones has won our Mythopoeic Fantasy Award twice.