Life, the Universe and Everything, by Douglas Adams. The Hitchhiker's Guide to the Galaxy series.

Part three of the Hitchhiker's Guide to the Galaxy trilogy of five.

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.