Don't Panic: collected together in the Hitchhiker's Guide Trilogy are the five titles that comprise Douglas Adams' wildly popular comedy science fiction series, with a special introduction from the author himself.

The Hitchhiker's Guide to the Galaxy
One Thursday lunchtime the Earth gets unexpectedly demolished to make way for a new hyperspace bypass. For Arthur Dent, who has only just had his house demolished that morning, this seems already to be more than he can cope with. Sadly, however, the weekend has only just begun, and the Galaxy is a very very very large and startling place.
The Restaurant at the End of the Universe

When all questions of space, time, matter and the nature of being have been resolved, only one question remains - 'Where shall we have dinner?' The Restaurant at the End of the Universe provides the ultimate gastronomic experience, and for once there is no morning after to worry about.
Life, the Universe and Everything
In consequence of a number of stunning catastrophes, Arthur Dent is surprised to find himself living in a hideously miserable cave on prehistoric Earth. However, just as he thinks that things cannot possibly get any worse, they suddenly do. He discovers that the Galaxy is not only mind-bogglingly big and bewildering but also that most of the things that happen in it are staggeringly unfair.
So Long, and Thanks for All the Fish
Just as Arthur Dent's sense of reality is in its dickiest state he suddenly finds the girl of his dreams. He finds her in the last place in the Universe in which he would expect to find anything at all, but which 3,976,000 people will find oddly familiar. They go in search of God's Final Message to His Creation and, in a dramatic break with tradition, actually find it.
Mostly Harmless
Arthur Dent has settled down on the small planet Lamuella and has embraced his role as a Sandwich Maker. However, his plans for a quiet life are thrown awry by the unexpected arrival of his daughter. There's nothing worse than a frustrated teenager with a copy of the Hitchhiker's Guide to the Galaxy in their hands. When she runs away, Arthur goes after her determined to save her from the horrors of the universe. After all - he's encountered most of them before . . .