Before Lifetime movies and afterschool specials told us that hitchhiking was dangerous, a lot of people got around by sticking their thumbs out. Seriously, just watch old movies: people used to hitchhike all the time in back in, like, the 1930s, and it wasn't likely to end with a serial killing. So it's not totally surprising that there really were travel guides aimed at cheap travel that were titled, for instance, The Hitchhiker's Guide to Europe, first published in 1971.
(Another book in that genre of cheap travel guides was Europe on Five Dollars a Day; but as Adams notes in an introduction in the Ultimate Hitchhiker's collection, he didn't have that book in the '70s since he wasn't in the five dollars-a-day tax bracket.)
Adams is clearly playing on the idea that you can travel around the galaxy cheaply, on "less than 30 Altairan dollars a day" (1.66), which is funny, because we usually think of space travel as something that costs a huge amount of money, the kind of thing that governments had to spend years and billions of dollars doing. Of course, this was in the 1970s, when billions of dollars was a lot of money; nowadays, every billionaire seems to want to get into space to mine asteroids just for fun. But we digress.
So by combining two thoughts that don't usually go together—hitchhiking and the galaxy—Adams is signaling that this book is going to be a little different from what we'd expect: it's going to take a serious issue (space, the meaning of life) and give it a comedic tweak. The title is a perfect signal of this book's mixed genre: comedy and science fiction. It's truth in advertising. How amazing is that?
Also, we have to add one more thing: note that "hitchhiker's" is singular—this is a guide for one hitchhiker. We guess that makes sense, since hitchhikers don't often travel in packs. So this is a guide for a single person alone in a wide, uncaring universe—and yet, let's note that the characters here are rarely alone. Arthur has Ford, Zaphod has Trillian, Benjy mouse has Frankie mouse, and so on. So the book's title promises one hitchhiker—but it's a big lie. So much for truth in advertising, unless the point is that it's hard to hitchhike totally alone, because, you know, if there's no hike to hitch, you're not going to get very far, and all that.
(Note: We grew up on the traditional American spelling of the title: "Hitchhiker's." But you will also find "Hitch-Hiker's" and "Hitch Hiker's." We pray we never see "Hitch-hiker's.")
(Subnote: If you look online, you will find some people refer to it as "H2G2"—that's two H's for "Hitch Hikers" and two G's for "Guide" and "Galaxy." But we only use that in emails and text messages—never in papers.)