The Salmon of Doubt is a collection of previously-unpublished material by Douglas Adams, published after the author's death in 2001.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 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 extremely short and only gives 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 half a missing cat, receiving large amounts of money from an unknown client, and then flying to the United States, although Adams had not yet decided where to, and at this point the story slows down and ends. As promised in the beginning, we do not go through a rhinoceros.

Early on in the book Dirk Gently dials his own phone number, and answers his own call. This may have been foreshadowing some sort of time travel later on.

One of the interviews reprinted in the book reveals that Adams did not like the ending he wrote to The Hitchhiker's Guide to the Galaxy "trilogy", and realised that the ideas he had been working on for Salmon of Doubt might work better as a sixth book in that series. 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 forms a link between the two series, or that the text was in a state of metamorphosis from one to the other - but no one will ever know. It is also speculated that the woman that is in a relationship with Thor might be Trillian, based on a scene in Life, the Universe and Everything, although there is also a candidate in the previous Dirk Gently novel, The Long Dark Teatime of the Soul.

• ISBN 1400045088

Early editions of Dirk Gently's Holistic Detective Agency, printed circa 1988, have a blurb on the cover reading, "The Dazzling Bestseller By the Author of The Salmon of Doubt".