This is a book on suicide, which is what Albert Camus regards as the only important philosophical question.

Life is absurd. How do we deal with the inherent meaningless of the universe?

1. Kill ourselves.
2. Philosophical suicide: acquire meaning from external sources (e.g. religion, etc.)
3. Live with it.

Of course, Camus believes the 3rd option to be the only worthwhile one.

This is also what leads him to the eventual conclusion (literally, the last sentence) of the book:

The struggle towards the heights is itself enough to fill a man’s heart. One must imagine Sisyphus happy.