I got "How to Make Sewing Patterns" by Donald H. McCunn. It's pretty basic, but decent. The first section is how to make a sloper to your measurements (which I skipped. The Big4 all have sloper patterns you can get and alter to fit you perfectly- giving you the added benefit of knowing exactly what alterations you need when using a pattern from that company.)
The second section, however, was the most useful for me when I was getting started, since it deals with how to alter patterns. Changing seam locations, darts, adding or removing fullness, and how to make different types of necklines and collars.
The third section deals with design elements of garments, like the fullness in a skirt, plackets, pockets, cuffs, sleeves, etc. etc.
The instructions for how to make a dress form are pretty crappy- a duct tape dummy is much better, but I liked the book overall and still refer to it sometimes.