I like books that are inspirational. I find most how-to books to be too simple or really not my style. Lark Books 500 Series is very exciting, and they also publish a TON of very good craft books in practically every medium imaginable - some how-to, and some showcasing works of artists. larkcrafts.com - but you might want to hide your credit card first
I also really like the Dover clip art books that come with a CD - I've collected several and I've used images on LO's (both digi LOs and I've printed images for paper LOs), in card-making, and random other craftiness.