I received my copy of Janet Szabo's Aran Sweater Design book yesterday in the mail.
While the previous edition of this book was full of good information, the latest edition is fabulous. Something I'd recommend for every knitter's bookcase.
From the basics of what makes a sweater an "aran style", to yarn construction and why some yarns show cables better than others, to compensating for cable splay, to different construction techniques, this book covers it all. Being of a visual mind, I love the amount of visual aids to help describe techniques. The chapters are laid out in a logical order to walk you through the process step by step. There's sample patterns to illustrate some of the different types of sweaters and there's worksheets to help you do your own designs.
I think there are certain books that every knitter should own. This is definitely one of them.