From The invisible man:
His feet, save for socks of irregular open-work, were bare, his big toes were broad, and pricked like the ears of a watchful dog.
The sentence means this guy was barefooted, but I'm not sure what open means, there.
Openwork (or open-work) is any style of fabric that is made with holes included, such as crochet, lace doilies, or some forms of quilt. It's also used for metal structures that have holes, but I believe this is less common.
It seems odd to hear it used for socks. Socks are usually knitted, and I don't see any reason to knit holes into socks. I guess it's a metaphor - these socks were so ragged that they seemed to have been made as openwork.