The book Camus wrote after L'Etranger (the Outsider), in which he expounds his theory of the absurd.
This is radical stuff. Every teenager discovers for him/herself that life and all its social rules is a frustrating game against which you can rage but rarely win. Camus takes it a step further. He compares our daily lives with Sisyphus rolling his stone up the hill every day for eternity, only to see it roll back down again; and he analyses how we are to survive this puzzling, pointless strife we call life. And surprisingly, he finds a streak of hope.