How To Choose The Sex of Your Baby, by Landrum B Shettles and David M Rorvik goes into greater detail about the method discussed below (timing intercourse to occur before ovulation). The authors claim that, when used properly, this method is very succesful, but they also warn that trying for a girl is more difficult than trying for a boy because it's tricker to get the timing right.
Taking Control of Your Fertility: The Definitive Guide To Natural Birth Control, Pregnancy Achievement, and Reproductive Health, by Toni Weschler is a great book that discusses the Shettles method among other things. I had this book a long time ago, but I loaned it out and never got it back, so I'm not entirely certain about this, but if I remember correctly, it has much more information about charting your cycles to determine when you ovulate and might actually make it easier to follow the Shettles method than using the Shettles book alone.
Good luck!