This is probably like teaching grandma to suck eggs but my girlfriend bought me this book a couple of weeks ago and its changed my life! Well, changed the way I look at the guitar and really given me a leg up on song writing.
The amazon.com reference is here
What I like, apart from the clear chord diagrams, is the constant use of actual songs throughout. The writing style is light but consistent and informative. In short - if your stuck or just starting out I'd highly recommend it.