I know. There are a LOT of sock knitting books out there. I just wrote about another one last week.
But, if you're interested in taking your sock knitting to the next level, Wendy D. Johnson is your girl.
Toe-Up Socks for Every Body: Adventurous Lace, Cables, and Colorwork from Wendy Knits uses the same high-quality photography, clear illustrations and gorgeous patterns as Socks from the Toe Up and builds on that success in two ways.
Some of the patterns are a bit more advanced. Charts and chart-reading are well-explained for the knitter, and cables and lace abound - sometimes in the same pattern.
There is also a greater variety of sizes. "Every Body" means that there are some kids' and mens' socks mixed in.
Overall, a strong addition to any library.