I studied journalism in college, and I had a copy of the AP Stylebook, but I don’t think I ever really used it. I found Strunk & White much more helpful to teach grammar skills, if that’s what you’re need to learn. Being forced to transcribe pieces of that book my freshman year helped my grammar tremendously.
If you want to focus more stylistically, I think the best thing to do would be to read really good magazines. They’ll have the highest standards, and you’ll pick those up from reading the articles in them.