Sock Yarn Studio: Hats, Garments, and Other Projects Designed for Sock Yarn has everything I want in a knitting book.
Firstly, it has a great concept. I love working with finer gauge yarns, and like Carol Sulcoski's first knitting book, Knit So Fine, this one explores everything about sock yarns. There's a full introduction to sock yarn and how they are categorized and some of the best uses for each.
Second, the book has great designs. From simple, to moderately complex, the designs are innovative and beautiful. The instructions are clear and concise as well.
Finally, the book provides seemingly unlimited inspiration. Inspiration in ways to use patterning yarns, shaping for knee-high socks, color choices that are beautiful and unexpeceted (there is a stunningly beautiful baby cardigan in beige - showing that pastels aren't the be-all, end-all of colors for baby knits) and ways to exploit the versatility and abundance of a yarn I have explored a lot already in my knitting.
For any knitter, having this book in their library will provide them with a consant source of ideas and designs.