I was looking for the same thing a while back and a friend recommended a book to me called Photoshop: Down and Dirty Tricks (http://tinyurl.com/3hqdumj). For me it was the best advice because I prefer to learn by doing. The Amazon link I provided has a "look inside" feature so you can see how the tutorials are structured and decide for yourself if it would work for you.

It is meant for people who already know a little Photoshop but if you know enough to use Acorn you should be fine.