They Know It When They See It

An inmate in the South Dakota State Penitentiary has been denied access to art-instruction books because they contain images of unclothed human bodies.  (Original story here and here.)  While not everything that could be called an art book is a good art book, shouldn't we be doing everything we can to help felons improve their lives?  And isn't art one of the best things they can do while in prison?  Let us grant the prison wardens their claim that pornography worsens prison conditions; does that mean that all nudity is obscenity? (Scroll down to the concurring position of Mr. Justice Stewart.)