The Elements of Style (4th Edition) by William Strunk Jr., et al.  If you don't have this book, buy it immediately. It's thin, easy to read, and offers absolutely indispensable advice for writing clearly and effectively. It has been around for a very long time, because no one has ever produced a better one on the subject.

Story: Substance, Structure, Style, and the Principles of Screenwriting by Robert McKee. Robert McKee is one huge guru for screenwriters. His advice fits any genre or style of writing you'd care to attempt. I love reading his book because it's both useful and inspirational.

The Writer's Journey: Mythic Structure for Writers by Christopher Vogler. Every story needs a mythic structure. Think I'm off base? Read Vogler and you'll see that I'm not. Ever wonder just why that dratted Wookie had to howl at just that time in the first Star Wars movie? This book will give you the reason.

The Comic Toolbox: How to Be Funny Even If You're Not by John Vorhaus. Comedy can be learned! This wonderful news is expanded on at very useful length in Vorhaus's marvelous book.

The Complete Author's Guide to Heroes and Heroines: Sixteen Master Archetypes by Tami Cowden, Caro LaFevre and Sue Viders. This is the best book on character development I've ever seen. Period.