“An old soul is the last thing you would expect to find inside Justin Bieber,” an old entry on his Web site says. “But all it takes is one listen to the 15-year-old soul-singing phenomenon to realize that he is light years ahead of his manufactured pop peers.” Mr. Bieber, now 18 and as big a pop star as ever, is the model for the 11-year-old with an old soul in Teddy Wayne’s sad-funny, sometimes cutting new novel, The Love Song of Jonny Valentine.