I've talked about screenwriting "how-to" books here before. Short version: I'm not a fan. If you've never written a script or thought about how a story works, or if you're still new to all this and need guidance, then sure, most of them have common sense stuff that can make things clearer. But you don't *need* to read any of them.
Of course, people are still curious. They wonder if maybe the books have some secret, magic formula, a short cut to telling brilliant stories. So, to save you some time and money, here is a bullet-point summary of every screenwriting "how-to" book ever:
--1: Have a Beginning, Middle, and an End. In the Beginning, kick off the story in an interesting, exciting way, introduce all the characters (making sure they're interesting, flawed, with voices distinct from each other, snappy dialogue that sounds real, and their own specific goals and conflicts, especially the baddies), and show us what the main character wants, and the obstacles in their way. In the Middle, throw all the obstacles at them and see how they cope, while avoiding visible exposition ("As you know, my father, Dr Robert McFuckleberry, the eminent parapsychologist, went missing last year under mysterious circumstances"), working it into the dialogue and actions subtly, showing us what's going on instead of telling us. Similarly, don't tell us stuff in the action description that can't be seen on screen (Jack is a black belt in AssKickFu, and loves his mum), show it happening (Jack uses martial arts to kick a guy's ass for insulting his mum), because every scene should move the story on, or reveal character, preferably both. Halfway through the Middle, throw in a surprising twist that moves the story in another direction. In the final bit of the Middle, have everything go wrong, and make it look grim for the main character. In the End, show the main character summoning up their strength for one final battle, where they overcome all the obstacles, save the day (in a surprising yet inevitable way that was hinted at from the very beginning), and walk off into the sunset having learned something and grown as a person - that, or they tragically fail/die, but with a glimmer of hope for the future. Keep it all between 90 and 120 pages (a page is roughly equal to a minute of screen time), and make sure it's in the proper screenplay format (use Final Draft or Movie Magic Screenwriter if you have some spare cash, or the free CeltX or BBC ScriptSmart Word template).
--2: Er, that's it.
There, now you can just get on with telling good stories, and haven't contributed to the weird industry of "expert" script advice from people who've never written a script in their life. That'll be ten quid, please. Cash or cheque is fine. Cheers.
(Note: I'm aware all books are different, and may have good tips, etc etc, but you don't need any of them to write a good story. You can get solid, practical tips and techniques from working writers like John August and Bill Martell (who posts a great script tip every day, and also has a series of brilliant Blue Books that are incredibly useful), things they've used in their long careers. And re-read this if you want my recommendations on books about writing and the industry. If you really must read one of the how-to books, and need a more in-depth analysis of how stories work, try "Save the Cat", or "Crafty Screenwriting" - they're both short, reasonably priced, have some useful thoughts, and the authors actually wrote real movies and TV shows. None of the other ones have any extra insight. Although I'm totally with McKee on the need to stop all the shaky camera shit.)