The pattern does not suggest a specific yarn beyond worsted weight acrylic. The kit that comes with the book has a fuzzy brown yarn for this pattern.

In addition to the yarn and hook you will need:
2 black plastic eyes (8mm)
black embroidery thread
tapestry needle
two pieces of brown felt
small scrap of white felt
stuffing

From pattern description in book:
<blockquote> Brigitte’s amigurumi creations are all cute and creative, and this goofy, grinning beaver is no exception. Brigitte’s attention to the finest details carries this pattern straight down to separate fingers and toes. Slightly fuzzy brown yarn gives the beaver an irresistibly furry coat, and felt adornments bring the beaver to life with his signature tail and toothy smile!