Ew. Ew. I really hate that. Before Green Gables; Mr Darcy Takes a Wife; Scarlett... all these freaking fake sequels to books the authors themselves didn't feel needed them. It's freaking... legitimized fanfic. And in every one I've read, the author could NOT write like the real author, and in some cases completely screwed with the mythology and left me with a sour taste in my mouth, even though I knew they weren't the "real" histories of the characters.
Honestly - if you can write, create your own damn universe. And if you can't, keep your money-grubbing paws off classic literature.
I will allow an exception for Christopher Tolkien's work on his father's writings. But only because he stuck to the manuscripts as far as possible, knew his father's work and intentions as intimately as anyone could, and didn't make up freakin' Teenage Arwen stories for no good reason.