Unifying all incarnations of the Guide is the eponymous Guide itself -- a talking electronic resource for the frugal spaceman, jammed with critical info on every planetary system in the Milky Way, from dangers to be skirted (see the entry on the Ravenous Bugblatter Beast of Traal) to meals to be savored (see: The Restaurant at the End of the Universe). Thing is, most of the Guide is outdated and useless, a galactic Wiki gone to seed, which is why alien Ford Prefect comes to Planet Earth in the first place, to update the listing (old entry: "Harmless." Prefect's updated entry: "Mostly harmless.") The Guide acts as Greek Chorus, filling in backstory, clarifying offhand references made by characters, and often getting in the better zingers. Such as the Guide distinguishing itself from Isaac Asimov's "older, more pedestrian" Encyclopedia Galactica (the raison d'etre for the culture-cataloging Foundation) by touting the fact it is "slightly cheaper; and second, it has the words 'DON'T PANIC' inscribed in large friendly letters on its cover."
As a science fiction writer, Douglas Adams didn't use science fiction as a vehicle for thought experiments so much as he used it as a joke reservoir, skewering established tropes with "firmly crooked" observances, by turns droll, surreal, subversively philosophical. At one point, he seems to take an even deeper dig at Asimov's Foundation, describing a side effect of the Infinite Improbability starship drive, also a deus ex machina that brings characters together not of their own volition or by simple coincidence, but by some curious perversion of physics -- as if relationships between people were susceptible to the same laws that governed the relationships between atoms and molecules. Hari Seldon, take your mathematical Psychohistory mumbo-jumbo and stuff it.
As technological prophets, SF writers are by their own admission correct about as often as broken clocks -- but an informed wisecrack, a sharply observed human foible, that's a tool that will cut to the bone for all time. Just ask Voltaire, Cervantes, or Alfred Bester. Adams favorite target for dissection: Bureaucracy. And his skill is uncanny across all formats of the Guide, revealing finally the Universe we live in, one not so much merely indirectly hostile to the human race as it is likely to gleefully strangle it in red tape:
The President [of the Imperial Galactic Government] in particular is very much a figurehead -- he wields no real power whatsoever. He is apparently chosen by the government, but the qualities he is required to display are not those of leadership but those of finely judged outrage. For this reason the President is always a controversial choice, always an infuriating but fascinating character. His job is not to wield power but to draw attention away from it. ... Very very few people realize that the President and the Government have virtually no power at all, and of these few people, only six know whence ultimate political power is wielded. Most of the others secretly believe that the ultimate decision-making process is handled by a computer. They couldn't be more wrong.
Which is when those large, friendly letters really do come in handy: Don't Panic.