The Hitchhiker's Guide To The Galaxy is a great series. The last book is a bit of a downer ending, but I see it as him saying, "Fuck this, I'm done," which I would be too after writing four completely nonsensical books about the same character.
Oh, I've been meaning to get back to those... Although I hear that the downer ending was somewhat unintentional as I think he meant to get back to it but he died before that... So it's like... A double downer ending.
Not likely IMO.
1977 Douglas Adams has Simon Brett sign the contract for his radio series "Hitch-hiker's Guide to the Galaxy"
1978 The HHGTTG radio series broadcast for the first time
1979-1982 The radio series is heavily adapted and released as a book trilogy; all the material co-written for radio with John Lloyd was removed.
1984 After the three previous books became bestsellers, he added "So long and thanks for all the fish" which iirc is mostly a love novel.
1991 Doug Adams marries Jane Belson
1992 After the four previous books had become bestsellers he added the fifth book "Mostly harmless". By this time he was into environmentalism more than scifi.
1994 daughter Jane Adams born
1999 Adams moves to California to support the movie, which iirc fails at the box offices
2001 Douglas Adams dies
IMO the first three parts of Hitchhiker are one "thematic block", he later wrote a good addon to make money (So long and thanks for all the fish), and then another botched addon because he needed money (Mostly Harmless sucks).