'''''The Hitchhiker's Guide to the Galaxy''''' is the trilogy-in-six-books by Creator/DouglasAdams, with the sixth book being written by ''Literature/ArtemisFowl'''s Creator/EoinColfer.

The first book, ''Literature/TheHitchhikersGuideToTheGalaxy1'', was adapted straight from the radio shows. It covers Arthur Dent's last day on Earth, meeting with the other characters, questing for the legendary planet of Magrathea, and the story of Deep Thought.

The second, ''Literature/TheRestaurantAtTheEndOfTheUniverse'', also came from the radio version, although with many more changes and a shifting-about of the order of events. These first two books can, in many ways, be thought of as halves of the same story, in a way that the sequels aren't. In ''Restaurant'', the characters visit Milliways, the titular establishment at the rear end of time, Zaphod and Trillian attempt to discover who truly runs the universe, and Ford and Arthur end up on a spaceship full of useless people which crashes into prehistoric Earth.

The third book, ''Literature/LifeTheUniverseAndEverything'', is the most conventionally adventure-ish book of the series; not surprising, since it was adapted from an unused ''Series/DoctorWho'' script. Ford and Arthur get pulled back to modern-day Earth, pre-explosion, where Slartbartifast enlists them and, eventually, the rest of the cast to stop the machinations of the xenophobic Krikkitmen, who, at the dawn of galactic civilzation, were responsible for the bloodiest war the universe has ever seen, but who were [[Main/SealedEvilInACan sealed in a slow-time bubble]]... until now.

The fourth book, ''Literature/SoLongAndThanksForAllTheFish'', is, on the other hand, probably the most character-based of the series. Arthur returns to an unexpectedly-resurrected Earth, but after his adventures among the stars, he's just as [[Main/FishOutOfWater out of his element here]] as he was when he first hitched a ride on a spaceship.

The fifth, ''Literature/MostlyHarmless'', is a dark romp through the corridors of probability. The Guide has been taken over by the Vogons, and Arthur has lost his love and has settled in as a sandwich-maker in a primitive tribe on a faraway planet. But then Trillian shows up with a surprise -- a teenage daughter, conceived with Arthur's donated DNA.

A sixth book, ''Literature/AndAnotherThing'' was written by Eoin Colfer, author of the Literature/ArtemisFowl children's novels, and published in October 2009. Starting where ''Mostly Harmless'' left off, the tone of the book in general is much lighter and removes the [[DownerEnding downer ending]] the series ended with.

For all versions of the story, see ''Franchise/TheHitchhikersGuideToTheGalaxy''.
!!''The Hitchhiker's Guide to the Galaxy'' book series provides examples of the following tropes:

* DownerEnding: ''Mostly Harmless'' ends with [[spoiler:all the main characters arriving on Earth and being erased from existence by the Vogons]].
* EarthShatteringKaboom: The most understated one in literature.
* EvolutionaryStasis: It is mentioned that the Vogons stopped evolving shortly after they stopped being an aquatic species. The planet Vogsphere has created many other life forms far more appealing than the Vogons, which they inevitably destroy for their own amusement.
** To lampshade it, the book even states that evolution threw up its metaphorical hands in horror at the sight of the Vogons in daylight, refused to let them evolve again, and produced the other, amazingly beautiful creatures of Vogsphere in compensation for the Vogons.
* [[ICantLookGesture "I Can't Look" Gesture]]: a variant; "Peril sensitive sunglasses" instantly turn completely opaque if anything dangerous or threatening appears to spare you the sight of it.
* ItRunsOnNonsensoleum: The series' phlebotinum runs entirely on RuleOfFunny. Among other things, we have the Somebody Else's Problem field, a [[StealthInSpace cloaking device]] that weaponizes WeirdnessCensor by making people ignore it. There's also the guy who built a starship powered by bad news, but nobody wanted it to show up.
* ShoutOut: To the PinkFloyd song "Set The Controls For The Heart Of The Sun." It's mentioned that "the loudest group of all times," called Disaster Area destroyed a spaceship directing it into a star. This shout out is not too surprising, considering that Adams was a friend of PinkFloyd guitarist David Gilmour.
* SingleLineOfDescent: Subverted with a minor character who's "a direct male-line descendant of Genghis Khan", who dies along with the rest of the Earth and has no further bearing on the plot.
* TimePassageBeard: Arthur Dent grows one of these when he is trapped on Earth 2 million years ago
* TrilogyCreep: "[[LampshadeHanging The increasingly inaccurately named]] ''Hitchhiker's'' trilogy"
* UnluckyExtra: Agrajag is an unfortunate soul that happens to reincarnate into incidental creatures that Arthur Dent accidentally kills (a pot of flowers, a fly, etc...). Agrajag eventually becomes aware of his past lives and becomes more and more spiteful toward Arthur until his dislike actually materializes into the "Cathedral of Hate", to which he eventually abducts Arthur; Arthur Dent proceeds to accidentally kill him while escaping. Thanks to the vagaries of time travel, Arthur manages to accidentally cause his death ''again'' two books later.
** In the Cathedral of Hate Agrajag, who had already experienced this death, [[TimeTravelTenseTrouble realized Arthur hadn't done yet]].