How do you know what "philosophical literature" is? Easy-peasy: it's not easy-peasy. In fact, it leaves you scratching your head and wondering (uncomfortably) "What is life, maaaan?"
The Stranger is philosophical literature at its baffling best—it uses a fictional story to promote or explore one specific philosophy: Absurdism, in this case. You're going to hear a lot of people saying that The Stranger is existentialist fiction, and then you're going to hear a lot of other (rather angry) people railing about how, no, Camus was adamantly not an existentialist.
So here's the thing: very few "existentialist" philosophers were willing to accept the label of "existentialist," starting with "The Father of Existentialism," Søren Kierkegaard, who basically went to his grave declaring that hey, he wasn’t an existentialist.
The reason it's hard to get any one definition of the philosophy is that there isn't a clear one: different thinkers promoted different ideas, and Existentialism evolved and morphed back and was generally the Mystique of philosophy. The point being, just because Camus said he wasn't an existentialist, that doesn't mean we can't talk about his novel in the light of both Absurdism and Existentialism. (This, of course, gets into a much larger and very fascinating argument as to whether a piece of literature should be analyzed with regards to the author's intent, or whether, once it leaves the pen/typewriter/screen, it stands on its own. But that's another issue.)
So most of this module talks a good deal about Absurdism. The skinny, as you've probably heard by now, is that the world lacks order and meaning, so looking for it is totally futile. The world is absurd and without logic. Great. So far, this is right on par with the brand of Existentialism at the time—Jean-Paul Sartre's brand, that is.
(Gossipy aside: Camus and Sartre were good buddies at first, but they differed in their views of the world. They both rejected religion and determinism, but Camus was more of a fan of man—he was a humanist—and not willing to sacrifice morality as a fundamental concept. Sartre was more focused on notions of choice and metaphysical being.)
But enough of the abstract brouhaha; let's talk about the nuts and bolts of The Stranger. It's easy to see the Absurdist aspect—Meursault's conclusion at the end of the novel is the main tenet of that philosophy. But let's see where Sartre’s Existentialist influence peeks in and where it is challenged.
Let's start with the issue of actions. Sartre believed that man was only his actions. Existentialism was based on the principle that existence precedes essence; you define yourself at each given moment by acting at each given moment. Now check out the courtroom scene. Meursault's attorney asks whether the man is on trial for killing the Arab or for burying his mother without emotion. The prosecutor responds that "between these two sets of facts [exists] a profound, fundamental, and tragic relationship."
In many forms of Existentialism, there is no relationship between these actions. Meursault defines himself by burying his mother, and again, later, defines himself by killing the Arab—but there is nothing linking them, no essence defining Meursault other than that created momentarily by his actions.
But perhaps Sartre's most important idea was that of radical personal freedom—the freedom to choose. We don't know about you guys, but we see a lot of that coming through at the end of The Stranger, especially in those last few lines. Meursault declares that all he has left now is to wish for a crowd of spectators—but he doesn't. One explanation for this (and we go into more detail in Meursault's Character Analysis) is that the point isn't for Meursault to feel less alone—it's that he can choose whether or not to be less alone. That he is able to choose, that he is aware of this ability, and that this is what defines his revelation, sounds a lot like Sartre.
So that—phew—was a long explanation for why the genre is "Philosophical Literature." But philosophy kind of demands that you spend a bit of time mulling it over; it's what makes philosophy philosophy (and makes philosophy majors universally the brooding type).