I'll just come out and say it: I love Grammar Girl. If you haven't been to her website, it's a great place to have fun learning about grammar and usage. Yes, that's right: I just used the words "fun" and "grammar" in the same sentence.
Grammar Girl, a.k.a. Mignon Fogarty, has a regular podcast discussing grammar and usage, and she also posts articles for those of us who prefer to read her tips. Her book is more of the same (in fact, I recognized some of it from the website) but all in one handy, well-designed package.