I'm reading the book I bought yesterday.
WebObjects Web Application Construction Kit, by George Ruzek
I'm really enjoying it so far, and I think it's very approachable for anyone with Java, OOP and RDBMS/SQL experience. I have noticed one very little problem with the book. I'm not sure if it would confuse anyone new to WO, though it did confuse the first person I met that had read the book.