->-- '''DON'T PANIC.'''
''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. It began in 1979, as an adaptation of the radio series of the same name, also written by Douglas Adams, but eventually diverged from and expanded on the plot of the original. It's arguably the best-known version of the series.
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. It leads directly into the next book.
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 civilization, 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. He attempts to solve the mysterious disappearance of the planet's dolphin population alongside his new girlfriend, Fenchurch, who is implied to be the woman featured in the first pages of the first book.
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. Its creator felt it was too strongly coloured by a bitter breakup he had undergone at the time, and intended to write a sequel, but due to his infamous procrastination, died before completing anything tangible.
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 [[LighterAndSofter lighter]] and removes the [[DownerBeginning downer beginning]] the series ended with. There is some controversy as to whether it lives up to the main series, and is [[FanonDiscontinuity considered non-canon by some fans]].
For all versions of the story, including the TV Series, Radio Series, Video Game/Text Adventure, Film, Theatre Plays and Comic Series, see ''Franchise/TheHitchhikersGuideToTheGalaxy''.
!!''The Hitchhiker's Guide to the Galaxy'' book series provides examples of the following tropes:
* DownerEnding: In ''Mostly Harmless''. [[spoiler:A malevolent plot by a reality-warping entity causes the Guide to be taken over and robbed of its soul by an evil publishing conglomerate, the Earth to be destroyed in every single timeline, and all the regular characters except Zaphod to be killed. Adams later regretted this and intended to retcon it out in a sixth novel, but his real-life death intervened.]]
* 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 it's 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.
* HappyEndingOverride: Between ''So Long and Thanks For All the Fish" and "Mostly Harmless", Arthur's true love Fenchurch mysteriously disappears in-universe (it's implied that a hyperspace-travel accident caused Arthur to accidentally jump into a timeline where she never existed).
* 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.
* LemonyNarrator: A staple of Douglas Adams' work. The fictional guide itself is also written in this style.
* TheMeaningOfLife: A machine is fed information to calculate the ultimate answer to life the universe and everything. The answer? [[NonAnswer 42]].
* ProphecyArmor: Arthur's encounter with Agrajag leaves him believing that he can't die until he's visited a planet called Stavromulos Beta, where he ducked to avoid being shot and the shot killed Agrajag instead. It turns out to actually be a nightclub on Earth called "Stavro Mueller's Beta".
* 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; thanks to the vagaries of time travel, it turns out that he can't kill Arthur because one of the deaths he wants revenge for hasn't happened yet (it eventually occurs in the final chapter of ''Mostly Harmless''). Arthur proceeds to accidentally kill him again while escaping.
* WhatOtherGalaxies: The Universe is mentioned several times (the Answer to Life, the Universe, and Everything, ''Literature/TheRestaurantAtTheEndOfTheUniverse''), yet no other galaxy is ever acknowledged. To be fair, the Restaurant at the End of the Universe is at the temporal end of the universe, not the physical end.