Read the first in the series first as the characters reappear and develop.
I'm enjoying them tremendously. They are odd and quite entertaining. Read the first and you will be hooked. See this post for the first two books, a blurb from amazon, and a link to a NY Times story on the author.
My best guess is that the tree socks were made by Knotty by Nature or Beehive, knitting & fibre arts stores. But I'm not sure and I kind of like the mystery!