There are two novels currently out. They elaborate a little on some side stories/back stories. The first novel has a story with Dino's days with Reborn, Mukuro taking over Kokuyo middle school (it's totally JUST AS PLANNED), Bianchi and Shamal working on a job together, and Haru and Bel going out for sushi at Yamamoto's place. The second one has stories on how Ken/Chikusa met Nagi, Xanxus's return to the Varia after being popsicled, I-pin and Hibari, and Gokudera's younger days (involves the 9th). There's another sidestory included in the guide book about the young boy Mukuro posessed at the end of Kokuyo.
God, I'm such a Reborn fag.