There are hundreds of books on writing: how to write for the web; how to punctuate; how to communicate more effectively; how to be a copywriter; how to improve your grammar; and how to write shorter sentences.
As with any field that remedies a lack in general education and answers a business need, there is a lot of conflicting advice from writers with varying standards and different motives. Here's a list of the some of the most useful books around; written simply, using good examples. Some are also quite funny.
All of these books should be available from the Amazon website. Please let us know if you have any books that you would like to recommend, and why.