I am not a lawyer, but I believe that as long as it is viewed as art, you should be ok. You are allowed to display recognizable people in an art gallery (that includes selling the prints as well), so as long as your book's main purpose is artistic (showcasing your work), I believe you should be ok. In Russ' case, the kid's picture would be used for a promotional brochure, thus commerce, rather than art, and Russ was right in not letting them use it. Again, just my non-legal, layman opinion.