I went to a couple of my favorite websites that have reliable, free patterns, but struck out looking for afghans. Both do have books and/or patterns, but you'll have to buy them and hope that they're what you want.
Knit Picks generally has books on sale, and their yarn is very well priced:
www.knitpicks.com Elann has TONS of books and more yarn than you could try in a lifetime. They had several afghan books:
www.elann.comAnother option for afghan patterns is Michaels or Hobby Lobby. I remember getting a Leisure Arts book at one or the other some years ago that used two strands of yarns together--very cozy and very fast!
Hope this helps,
Becky