I don't think there are any writing books out there specifically on YAs right now. I have heard that Lauren Barnholdt (I think she posts on these boards sometimes) and her agent Nadia Cormier have a How to Write YA book coming out in December...2006. (Sorry if I got any of that info wrong - I tried to find my source for it but I couldn't.)
I agree that How to Write Children's books are pretty much useless when it comes to writing YA. You can find some specific info on verla kay's message boards - although a lot of the people there write for a younger audience, there are some people who focus on YA. Also smartwriters.com has some interesting articles - again, you have to sift through a lot of things, but there are some articles on YA.
Another suggestion might be to look on websites of some of the YA authors that you like - sometimes, but not always, they have essays about writing or their process, etc. Some of these might include helpful info. Nancy Werlin has a couple of interesting articles on her site, and I think Alex Flinn does too. Also interviews - actually, it might be an interview with Nancy Werlin that I'm thinking of, where she talks about how her first book started out as an adult novel, but then she changed it to YA, and she talks about both how and why she did that.
Do you have specific questions about writing YA or were you just looking for a more general overview? If you have specific questions, let me know, and I'll try to see if I can think of places that might have some answers or suggestions for you.
