The Salmon of Doubt is a collection of previously unpublished material by Douglas Adams, published after the author's untimely death in 2001. It consists largely of a compilation of essays, most of which have a technological edge, but its major selling point is the inclusion of the incomplete novel on which Adams was working when he died (and from which the collection gets its title).
The actual manuscript of the proposed novel is, unfortunately, extremely short and only gives us a glimpse of what The Salmon of Doubt would have been. It is composed of the best content out of several drafts (as were many of Adams' books). The existing plot involves Dirk Gently, the detective protagonist of two earlier Adams novels, refusing to help find the missing half of a cat, receiving large amounts of money from an unknown client, and then flying to the United States. The sign reading "Gusty Winds May Exist" coupled with the fact that half the cat was missing suggest that the cat may be an actual representation of "Schrödinger's cat", which was referenced in Dirk Gently's Holistic Detective Agency.
After refusing the case about the missing half of a cat, Dirk pays a visit to Kate Schechter (who had first appeared in The Long Dark Tea-Time of the Soul). Dirk tells Kate that prior to the potential client, he had been so bored that he had started a habit of dialing his own phone number, and discovered that he'd answered his own call. This may have been foreshadowing some sort of time travel later on.
In one of the interviews reprinted in the book, we learn that Adams did not like the ending he wrote to his Hitchhiker's Guide to the Galaxy series, and that he was thinking of continuing the trilogy in the Salmon of Doubt. In the book, Dirk follows around a ginger haired actor. Ford Prefect is described as having ginger colored hair, and his disguise during his stay on Earth includes his being an out of work actor. It is possible that this is him, and that it is the link between the series that Adams promised, but no one will ever know.