There are bot knitting and crochet patterns for a slipper boot called a Mukluk(sp?). So if you are looking for more patterns for a kneehigh slipper boot do a good ol' seach for mukluk. I have 2 pairs and they are super warm and cozie.
You have to search pretty hard to find the patterns though. Here is a photo of similar ones.http://www.nursinghomeapparel.com/shop/product744.html