I would buy the book. Lucy Neatby's DVD's are wonderful and a very good teaching tool. but they don't have any patterns on them. After i bought both DVD's i went out and bought the book to get the patterns so i could try my first sock.
Actually, her DVD's are the reason i even have the bravery to try to make socks and use dpn's (scary). I think they are great and her book is great. She has patterns in it from beginner to "Sock Goddess".
If i were you, I'd get both.