It's not just And Another Thing.There is a positive slew of sequels around. In children's books we had last year Geraldine McCaughrean's Peter Pan in Scarlet and now a "new" Winnie-the-Pooh book and indeed Hilary McKay's sequel to Frances Hodgson Burnet's A Little Princess. (That one's called Wishing for Tomorrow and I would have reviewed it here if Hodder had responded to my request for a copy).
In the world of adult books, sequels and prequels to the classics from Jane Austen to Daphne du Maurier have always been popular. (I blame Jean Rhys myself and the wildly overrated Wide Sargasso Sea, which opened the floodgates). Recently even Sebastian Faulks has produced a "James Bond."
But how different is this from fan fiction? It does the same thing, using characters and settings already provided by the first author and creating new plots. So it's only the quality of the pastiche that is an issue. Sometimes it's very successful:I'm a huge fan of Leon Garfield's completion of The Mystery of Edwin Drood, which is almost a sequel, and Gilbert Adair's very stylish Alice Through the Needle's Eye. And I remember how much my daughter loved Silver's Revenge, Robert Leeson's sequel to Treasure Island.
I had to stop reading my own fan fiction on www.fanfiction.net because it was having a disastrous effect on my own writing! I was writing a pastiche of myself. So I drop in every now and again to see things like which characters from Stravaganza are attracting the most interest and so on. But I don't read it.
And I've never written a sequel to anyone else's fiction, though I had a very good idea for Five Grandchildren and It but was sort of beaten to it by Helen Cresswell. I imagine editors all over the UK are eyeing up children's classics and thinking what to plunder next.