Googling child porn in literature will get you a few hundred hits, the newest are about the Supreme Court upholding laws against possession of material promoted as child porn even if it does not actually involve real children. That's the law passed to deal with virtual porn.
Again like most pornography rulings exceptions are made for works of artistic and social merit.
To answer your question
www.unh.edu/ccrc/pdf/CV96.pdf
www.wla.org/igs/ifig/principles.doc
laws.findlaw.com/us/000/00-795.html
www.cyber-rights.org/reports/uscases.htm - 50k
all deal with the shifting definitions of how child pornography is legally defined.