Well, there was a official sequel, Peter Pan in Scarlet
, which was written by Geraldine McCaughrean in 2006, which was authorized by the Great Ormond Street Hospital, the current rights holders. There was also a series of prequels published by Disney's Hyperion Books and written by Dave Berry and Ridely Pearson. The books in this series include Peter and the Starcatchers
(published in 2004), Peter and the Shadow Thieves
(2006), Peter and the Secret of Rudoon
(2007) Peter and the Sword of Mercy
(2009) and the still in progress Bridge to Neverland
. According to the Wikipedia page for Sword of Mercy, these book have not been authorized by the Great Ormond Street Hospital.