Death comes to Pemberley

Sequels should be written by the original author or not at all.

PD James is an excellent writer and capable of inventing her own fascinating characters, so why did she write a sequel to Pride and Prejudice?

I refused to read the book, which some say is dull, but I have watched the first episode of the TV series. It is atmospheric and tense, but what struck me forcible was that it could have been one complete novel, not a sequel to any thing else.