Story by McKee is the best screenwriting book I've ever read (and I've read many).
I think a lot of people completely miss the points in the book. Unlike other books which usually offer up a formula, Story teaches you principles. You have to have the kind of mind where you read the material, and then think it over on your own, reread it, understand it, but also let it carry you into your own thoughts where you can come up with your own ideas on any given topic.
Again, it won't teach you a formula where you read it and try to copy it into your future script. Doesn't work that way, nor should it. You need to really think and get it.
Get some other book for actual technical writing instuction however. There are many books out there that will teach you formatting and writing style and the like.
Add: read scripts and watch movies. My personal advice is to read a few scripts at first to get a feel for the style. After that, focus more on watching as many films as you can.