I will just chalk anything like this up to non-canon attempt at money making. To me it is no different than if someone said they were writing a sequel to Dante's Inferno or or Hemingway's The Sun Also Rises.
I wonder if this will expand on the stuff in Salmon of Doubt, or if this will go off on its' own direction? The unfinished story in Salmon was supposed to be the direction Douglas Adams was planning to take for the next, and really really real final Hitchiker's book.
Wait a moment - I think I read Salmon of Doubt (of course, unfinished, but with several further material from and around Adams), and it was a Dirk Gently Book, not a Hitchhiker book.
Sure, and both Mostly Harmless and the five novels of the Merlin cycle stand as testament that even original author needs to know when to tell his publisher "no, that's done". Closure's not a bad thing. Always leave'em wanting more. Just ask George Lucas or the Wachowski Brothers.The Amber "prequels" should stand as testament to the folly of allowing others to dabble in the original author's playground.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.