@FireMadeFlesh, Poetics is about literature and writing, especially tragedy and epic poetry. It talks about the poetic use of language and the elements of tragic drama, including the classic definition of the tragic hero. It has the useful concept of the “implausible possible” and the “plausible impossible”—an idea that I still find relevant nearly 40 years after studying this work in a literary criticism seminar.
If I were about to explore the Existentialists, I would first read up a little bit on Jean-Paul Sartre and Albert Camus, noting Sartre’s career of self-promotion as a celebrity intellectual, his relationship with Simone de Beauvoir (nearly as famous a couple as their namesakes here on fluther), Sartre’s coining of the label “existentialist” and applying it to Camus as well as himself, and Camus’s assertion that he was no such thing but rather an “absurdist.” Nonetheless the term stuck and Camus is invariably grouped with Sartre under this label.
Then I would probably venture forward by reading several novels of Camus, whose philosophy is expressed primarily through fiction. There is plenty of information out there to guide you—here is one source I like, even though, or perhaps because, it is personal and idiosyncratic.
There are also the relatively accessible plays of Sartre, notably Huis Clos, as well as his longer works of fiction and his essays, foremost and definitive among them the famous “Existentialism Is a Humanism” (1946), originally delivered as a lecture in 1945. This can be found in Walter Kaufman’s Existentialism from Dostoevsky to Sartre as well as other places.
If I took to it after that, I might use Kaufman’s book as an entrée to the thinkers leading up to this movement, including Dostoevsky, Husserl, Kierkegaard, Nietzsche, and others. This is an old (1956—but in print only ten years when I bought my copy) and limited anthology in a very thickly populated field, but it’s not a bad starting point.
In Sartre’s later years he had a political axe to grind. Camus died before having any later years. This movement continues to hold ground in the philosophical arena, and its lineage can be traced to Kant and his predecessors even though (like many younger generations) it takes a drastically different road from that of the old guys. The great issues remain constant.