Personally, I've read quite a few screenwriting books, and I believe there is some value in them, but you can't expect to read books and become a career screenwriter. And Mazin is definitely right when he says,
"You’re far better off reading screenplays (lots of places on the internet to do that) and watching movies."
But I do like to read books, and as Terry Rossio put it in comment #22,
"You might have overlooked one of the main reason why many people buy screenwriting books, from authors who don’t actually write screenplays.
Read the post here.
- Those who can't write, teach seminars (johnaugust.com)
- Reader Question: What is the best way to learn screenwriting? (gointothestory.com)
- Tips for Revising Action in Your Screenplay (brighthub.com)