If you're serious about writing screenplays, the best book for you is the one that gives you inspiration rather than fear or some other negative reaction. It’s possible you’ll pick up Story and think, “So much for screenwriting! I’m going back to writing free form poetry!”
It's good to find your own favorite screenwriting book, but it's probably a good idea to read or at least skim through them all. There are nuggets of wisdom each of these:
1. SCREENPLAY: The Foundations of Screenwriting by Syd Field. This book isn't exactly cutting edge, but it's solid. You may not be enlightened by Syd Field's classic, but you won't be disappointed, either. If you've written screenplays for a while, you probably already own this.
2. STORY by Robert McKee. If you don't want to spend thousands of dollars attending the famous McKee conference, pick up his book. Although a little dry and perhaps unnecessarily detailed, it offers such a wealth of info that it's easily worth the price.
3. CREATING BLOCKBUSTERS! by Gene Del Vecchio. Although this isn't a standard classic for screenwriters, it offers a perspective that is rarely addressed. Namely: it helps you develop story ideas that are genuinely marketable -- and it does this without completely devaluing the fact that stories are important cultural artifacts (works of art).
4. THE ART OF DRAMATIC WRITING by Lajos Egri. While focusing on the construction of plays, this highly regarded book is valuable to all types of storytellers, including screenwriters. If you've ever taken any screenwriting classes workshops, you'll know that this is a favorite among teachers.
5. THE HERO WITH A THOUSAND FACES by Joseph Campbell. Want something a little more academic? This is it. This book has probably had more cultural impact than all the other screenwriting books put together. In fact, it's the basis for many of the others. It's a bit dense but highly recommended.
6. SAVE THE CAT by Blake Snyder. If you've heard of only one screenwriting book, this is probably it. Some people swear by it; others hate it. I'd say it's a little overrated and the thrust of the book has been so widely disseminated that's it's almost unnecessary to read. Then again, there are those who swear by it...
7. WRITING MOVIES FOR FUN AND PROFIT by Thomas Lennon & Robert Garrant. The title and cover image say a lot about this book: it's a bit over the top. It's also surprisingly well written and includes a bunch of common sense hints and tips that I haven't seen anywhere else. If you just slogged through Joseph Campbell's writing, this light-reading masterpiece may be just want you need to read next to cure your aching and overloaded head.