I'm not a writer, per se, but have been an editor, proofreader, and project manager producing books for many years. I would highly recommend the Chicago Manual of Style, 14th ed. for anyone wishing to become a professional writer. Also, as mentioned by several above, Writer's Market is an outstanding source for markets, how to articles, etc. One caveat: Writer's Market is published annually, so it's usually more efficient and economical to visit your public library than to buy the book. (I think it was running somewhere around $35 US last time I looked.)

And long may you writers write, 'cause you keep me in business too! Best of luck to all of you.

Lin