The book in which I discovered a variety of Tolkien's art, not just his book illustrations, is the one this was scanned from. "J. R. R. Tolkien: Artist and Illustrator" by Wayne Hammond and Christina Scull. I bought my copy used through Amazon a few years ago. It's still available.

http://www.amazon.com/J-R-R-Tolkien-Illustrator-Wayne-Hammond/dp/0618083618/ref=sr_1_1?s=books&ie=UTF8&qid=1372465019&sr=1-1&keywords=J.R.R.+Tolkien%3A+Artist+and+Illustrator

Hammond and Scull's text is excellent at presenting and explaining the pictures.