This is where the murder comes in, happening at the beginning of the work though chronologically speaking it takes place in the middle. Marsault kills an invalid who all but asks him to do it after he has taught the protagonist the secret to happiness, which he cannot partake in because he lost his legs, and ends up taking enough cash to live out his days comfortably. For the rest of the book he learns to do just that, finally living it out just before he dies a horrible death delivered via tuberculosis. Ah the irony.
After finishing the novel, I described it thusly: "typical Camus, existential bullshit." Yep, life is pretty absurd. Camus always makes that pretty clear. But then again, Camus's work usually ends up being sort of uplifting, despite how bleak everything is on the surface level, because it teaches us the need the importance of living instead of droning on half dead. And believe it or not, A Happy Death is one of the author's happier novels in that it shows that his protagonist knows how to enjoy life in a way that we wouldn't consider too socially repulsive or anything. While The Stranger is the better book, in a strange way, A Happy Death actually illustrates that suicide isn't a logical option because we always have that ability to create our own joy, though we require enough time and money to do so. In his own words, Camus wanted us to use "our money to gain time," instead of using "up our lives making money." If only we could all afford the time.