The Abhorsen books are far and away my favorites. I've read just about everything else he's written with the exception of The Ragwitch; I think I'm in a minority for not really liking Shade's Children, though it's been eons since I've read it.

If, like me, you can't stand to wait for his forthcoming books (Clariel has been moved to a 2013 release, argh!), I'd recommend checking out The Keys to the Kingdom and The Seventh Tower--The Keys more than The Tower, I admit. I think Keys is the stronger series overall, though for my tastes he didn't quite stick the landing at the end of it.