People who read the book "Mostly Harmless" with a focus on the events of the book will probably end up disappointed. Adams continues his shift toward writing character centered novels (as begun in "So Long, and Thanks For All The Fish", and continued in the Dirk Gently novels).
The story focuses on three characters: Arthur Dent, the perenially confused Briton; Ford Prefect, the manic Betelgeusian; and Tricia McMillan, a BBC television reporter who, had she decided not to go back to get her purse when she met "Phil" at a party, would have become Trillian.
Adams presents Dent as a wonderfully tragic picture, and mirrors the beginning of "The Hitchhiker's Guide to the Galaxy" quite well. Dent is presented as a lost soul, desperately in search of a home and a place to belong. Of course, when he actually finds a place where he belongs and becomes comfortable, outside events tear his world apart. This is a theme that was present in three of the four previous HHGTTG books, but the presentation is probably the best in "Mostly Harmless" (and particularly intriguing is that Arthur's world is torn asunder by a person searching for the exact same thing as Arthur had: a place to belong).
The story of Tricia McMillan offers an interesting counterpoint to Arthur's troubles. Although she is a successful television personality, she is obsessed with the consequences of one of her past choices. Unlike Arthur, who is preoccupied with the process of finding a place to belong, Tricia is preoccupied with thoughts of what might have been.
Unlike his previous books, Adams allows the tragedy to come to a fitting end. The tone is certainly darker than the previous HHGTTG books, and the humor is perhaps less prevalent. Even so, it is a fitting end to a fine series.