My Princess Boy: A Mom's Story About a Young Boy Who Loves to Dress Up, by Cheryl Kilodavis
10,000 Dresses, by Marcus Ewert
Morris Micklewhite and the Tangerine Dress, by Christine Baldacchino
Jacob's New Dress, by Sarah and Ian Hoffman
A friend also recommended a great article from New York Times Magazine about gender fluidity in boys. I found it very informative.