I like So Long and Thanks for all the Fish, in part because of how sweet and optimistic it was. After finishing I then was shocked by how dark and depressing Mostly Harmless was.
One of the greater modern literary tragedies is the way Adam's said he wrote Mostly Harmless when he was in a very bad place mentally. He said he intended to write a followup to cap the series off in a lighter way but then died before he was able to.
