What is the connotation of “The Hitchhiker's Guide to XYZ”?
The exact connotations that individual authors (or their publishers) wish to convey in naming their book “The Hitchhiker's Guide to XYZ” is likely be a bit varied.
This is because, unlike titles of the form “XYZ for Dummies” which fall under the trademark of a single publisher (who curate/conform their collection to all be instructional/reference-books-for-beginners), the “The Hitchhiker's Guide to XYZ” template does not fall under one master trademark (and thus there is no single authority making sure everyone follows the same set of connotations).
That said, understanding some general connotations these authors/publisher's might be trying to convey probably requires a bit of cultural background.
I guess the titles are a reference to The Hitchhiker's Guide to the Galaxy.
You are correct; Douglas Adams's novel (based on the writing he had done for a BBC radio program) is the dominant reference for this snowclone/template. I say dominant, and not original, because Adams himself jokingly based the title off the then-quite-popular travel-guide “Hitch-hiker's Guide to Europe” (as DJClayworth notes in more detail in his answer). Even more confusingly, Adams's book (which is essentially a fictional narrative following the character Arthur Dent) often makes reference to an in-universe book by the same name (which is a bonafide travel-guide).
Adams's books went on to sell quite well to the general public; but the science-fiction settings and quirky, sometimes irreverent, often ironic, humor ended up cementing the book(s) as a staple in the literary canon of modern 'nerd culture'. (Which is why copycat titles so often appear in texts about Computer Programming and nearby STEM fields.)
So, what general connotations might these authors/publisher's be trying to convey?
Some might be using the 'nerdy reference' simply to sell more copies of their tech-oriented reference book by having a title that is memorable/hip/catchy to their STEM-oriented audience (no matter what writing style they use in the text). Others might use the reference as an indicator that their writing is "more casual than a textbook" in a similar way that “XYZ for Dummies” benefit from their title. Others still might be using the reference as an indicator that the author is going to make every attempt to be humorously as quirky, irreverent, and ironic as Adams was in his tome.
In general, though, it's probably safe to assume a typical instructional/reference-book somewhere in the ballpark of “XYZ for Dummies” to “(O'Reilly Media) XYZ” with a somewhat casual style.*
* Do read at least a few pages before purchase to get a feel for the author's tone/style; comedy writing (or lack thereof) in one's personal reference library is likely to be a bit polarizing.
As an aside: the movie version was quite bizarre (presumably by design). The book features a lot of wordplay, which - in addition to being very language-specific - can also be very hard to translate to film even for English-text to English-film (which I believe the film-producers tried to replicate by making the film even quirkier). Furthermore, condensing an entire book to a single film can be difficult in terms of time... and meant that a large amount of expository material was omitted entirely to eliminate some plotlines while other exposition was expanded to clarify main plotlines for the video audience.
Google seems to indicate that there are a number of Chinese translations of the book - though I have no clue how well any of these might convey the humor properly; my best guess would be that a Hong Kong translator would likely best understand the dry British humor that is so often present.