One good example of subtext in the book is the utter futility of everyday pursuits: Arthur was trying to save his house, but the entire planet is destroyed; Zaphod is trying to find the mythical planet Magrathea, but he finds that he is being manipulated by super-intelligent mice; the mice have spent ten million years waiting for the biological computer Earth to find the Ultimate Question (the Answer to which is 42) but the planet is destroyed five minutes before it was due to discover the Question.
"At this party," persisted Arthur, "was a girl... oh well, look it doesn't matter now. The whole place has gone up in smoke anyway...."
"I wish you'd stop sulking about that bloody planet," said Ford.
(Adams, The Hitchhiker's Guide to the Galaxy, Google Books)
Throughout the book, there is the underlying theme that because of the enormity of the universe, life is ultimately futile, and instead of spending time trying to make great and important changes, people should focus on their own lives, their family, and their friends. While this subtext is not primary, it persists through the series to the final book, which Adams wrote while suffering depression; every main character is killed and their journey ultimately means nothing.