Next to the Bible, the book that pops up most often in Dead Man Walking is Albert Camus' Reflections on the Guillotine. The guillotine here, of course, is the decapitation machine used during the French revolution; it's a method of execution. Camus' reflections on the guillotine are that the guillotine is a bad thing and you shouldn't execute people. So you can see where he and Prejean might get along.
Beyond that, Camus is a big deal atheist philosopher, with an "I am a big-deal atheist philosopher!" T-shirt from the Atheist Philosopher's Collective. If you're not Christian, or you think Prejean isn't a sufficiently Big-Deal Philosopher, Camus is there to tell you that you should oppose the death penalty anyway. A condemned man, Camus says, "is no longer a man but a thing waiting to be handled by the executioners." (2.82) In other words, it's not just Christians who say that capital punishment robs you of human dignity; important atheist philosophers say the same thing.
Camus isn't the only thinker referenced in the book; Prejean also references Susan Jacoby, for example, a scholar who writes on revenge. (7.7) All these citations are meant to buttress Prejean's point; they demonstrate that she has read and thought on the subject, and they show that other people agree with her. References, to Camus and others, function as authority; they help Prejean shore up her case that the death penalty is bad and should be abolished.