hi! I highly recommend 'The Glass Castle', by Jeannette Walls. It's a memoir in the style of Angela's Ashes, only less depressing. You'll never complain about your childhood again after reading that!
I agree that Water for Elephants was great, but I didn't like Sara Gruen's other books (Riding Lessons and Flying Changes) at all, because I found the characters so unlikeable, and the horse-related stuff was off. If you like fantasy, there's the Golden Compass series by Philip Pulman, although the writing is a bit dense and labored.
If you're looking for a horse novel I can suggest a new one, but be aware that it's blatant self-promotion, because I wrote it :) It's called Blaze of Glory, by m. Garzon. Actually since someone mentioned The A Circuit I have some gossip about that, if you're interested: the editor at Bloomsbury really liked my book, but then they got the manuscript that was 'co-written' by Georgina Bloomberg (Catherine Hapka actually wrote it. Another story there), so they understandably went with that one. As it was explained to me, she's the daughter of the mayor of NY and a wealthy socialite, so curiosity about her will sell books. It's not bad though, I read it. Very gossip-girly. If you're looking for a non-horsey YA romance, I really enjoyed If I Stay and the sequel Where She Went by Gayle Forman.
I'm going to look into the Sue Grafton series too!