According to How to Revive Another Author's Characters there is a "booming trend to sequels to other people's work." Including, but not limited to, a new Winnie the Pooh book and a sequel to The Little Princess. (Internet research suggests that should be A Little Princess, though I wouldn't know, myself, because as a general rule I try to avoid books with the word "princess" in the title. Not always, but if I can.) Also, Arthur Conan Doyle's estate has supposedly approved a series of books on the young Sherlock Holmes.
I would have thought Sherlock Holmes was a character who had pretty much been wrung dry. I was going to make the assumption that books about a young Sherlock Holmes would be directed toward child readers, but maybe I shouldn't.