The Elements of Style

Strunk & White’s Elements of Style is one of the best works explaining the rules of writing in the English language that has ever existed (and I’m pretty sure that sentence broke at least five of those rules). I’ve given this book to many a budding writer over the years. I’ve also recently noticed that it’s now all over the Internet, for free. For example, Bartleby has posted the 1918 edition of Elements of Style here.
XKCD's Elements of Style
If you haven’t read Elements of Style then I strongly recommend it. It’s short, concise and explains why that apostrophe goes in that one spot as opposed to the other. If you want to be a writer, this is one of your first stops on your journey.