I've got two writing books on my shelf here at work. "Elements of Style" and "On Writing Well." People who have no love for either make me frowny-faced. For learning the basics of clear writing, they're both good guides. If you learn the basics in these guides, you'll write sentences that are mechanically sound. Only when you've got those basics nailed down will you have the skills to begin to master writing beyond clarity and communication.

That said, they're not something that you read and have an "ah-ha!" moment of "here's how to write better!"

My best recommendation for improving your writing? Read. Find an author whose style you love, and read one of his or her books a half dozen times. The first two or three times, you will be busy enjoying what you read, but then once you've gotten past that, you will start to break down the mechanics of the writing. Why use these words? How is the writer pacing a scene? A plot arc? Learn the tools in a writer's toolbox, and then you'll find yourself improving the craft.