Sweater Design in Plain English by Maggie Righetti is an absolute must have. It's 300 some pages of great information, tips, and all the math you'll ever need to adjust/create your perfect sweater!
In my opinion, this book is a requirement for anyone who wants to modify a commercial pattern or make one from scratch. She also has a great sense of humor, which makes her books enjoyable to read. She also includes basic anatomy lessons, like the neck rests on the body at an angle, etc.
You can find it at Amazon or your LYS.
I just received my copy this week and was surprised at how much info it has. Great book