If you mean The Hitchhiker's Guide to the Galaxy, man, I have a completely different take.

I use that as one of two prime examples of a series that I love the first book of but steadily like less-and-less as the series goes on. The other example is Dune.

Hitchhiker's Guide to the Galaxy starts out funny. Okay, yes, black humor, but it's funny. And it gets steadily less-pleasant and grimmer over the course of the series. I'm not really enjoying it towards the last book or so any more.

Maybe a third series would be the Calvin and Hobbes comics, though I don't know if you can call that a series. Late Calvin and Hobbes, if you've ever read through a complete compendium, is very rarely funny, just kind of unhappy and cynical. The early and mid stuff, by contrast, is my favorite comic.

EDIT: Well, at least Watterson did leave it on a positive note with the final comic:

https://lemmy.today/pictrs/image/c45362c0-e85d-40b2-a54e-f94bd3308768.webp