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As a 'tribute' to Douglas Adams for the 30th anniversary celebrations (although the series itself is older than that, dating back to the radio play of a year earlier), this could have been quite interesting.
As a direct continuation of the series, however, I am less happy. There are situations where the author passes away in the middle of writing an extensive, serialised story. In these circumstances, with the narrative left unfinished, it is understandable that another author can be brought to finish the story. If the original author is aware of their illness, they can prepare notes and give their blessing ahead of time. Robert Jordan did this for Brandon Sanderson's conclusion of the Wheel of Time, and David Gemmell did this for his wife Stella to finish the Troy trilogy.
For a separate author to come in eight years after the first writer's death and brazenly write a new book in the first author's series is another matter. Yes, Adams' widow has given the project her blessing and yes, we know Adams regretted ending the fifth book, Mostly Harmless, the way he did and planned to one day write a sixth volume. But it still feels rather dubious, and the motives of the publisher are open to question (as they were with the Amber books of a few years ago and with the new Dune books now).
I'm sure it will be a success and Colfer is one of the very few authors who is even vaguely capable of matching Adams' style (the only other ones who could come close for me would be Neil Gaiman, Terry Pratchett and Rob Grant), but it still leaves a vaguely uneasy feeling of an author's work being mined without his permission purely for profit.
