On occasion, I stop my the bookstore and go to the reference section just to get a good grammar book. It doesn't necessarily improve my own everyday usage, sadly, but I almost always find them fascinating.

My latest find is absolutely fantastic; it's called Mind the Gaffe, by R.L. Trask, and it's basically a giant FAQ regarding writing style. The author is concise and, even better, entertaining. He does not hesitate to be rude about incorrect grammar. I've already learned a ton, and I'm not even past the letter A. I now know what a pleonasm is. :)

If you don't already own it (it came out in 2001, and was recently reprinted following the author's death), you can get it here: http://www.amazon.ca/Mind-Gaffe-Troubleshooters-Guide-English/dp/0061132209/ref=sr_1_2?ie=UTF8&s=gateway&qid=1202569252&sr=8-2 or here for Americans: http://www.amazon.com/Mind-Gaffe-Troubleshooters-Guide-English/dp/0061132209/ref=pd_bbs_sr_1?ie=UTF8&s=books&qid=1202569333&sr=8-1

I highly recommend it to anyone who needs to know when it is proper to use any one and when you should use anyone. :)