Heidegger's Being and Time
This may not be the pinnacle of Existential expression, but it certainly is the most formally thought out, and perhaps the densest and technical, depiction of the fundamental idea of being-in-the-world. Whilst Pascal was maybe the first to suggest the existentialist position; it was Heidegger in Being and Time who wrote "...existence is our essence..". This book is contra almost all of the philosophical tradition, from Plato on, and essentially prefigures pretty much all of the 20 century continental thinking.
What is he saying...? well, its so hard to get stable understanding from this text (perhaps this is the point) because it condenses earlier, more detailed, lectures; but...he is trying to ask the question "what being is it that asks the question about being?"--which turns out to be Dasein (Being-There)--this is similar to what Kierkegaard asked; and this being has all sorts of existential structures; like the idea of "thrown-ness"; that is we are thrown into the world at a certain time and place and a certain situation (this is what Sartre picks up in Being and Nothingness); we also have all these "for-the-sake-of-which's", which could be roles, which is how the world becomes intelligible (or is it understanding...?)
If you are fixated on Mind, the Real World, Correspondence, and subject-object distinctions then this may be your antidote. If you're keen, but cant see your way through 400 pages of terse German writing, you might want to try the lectures by Hurbert Dreyfus, perhaps one of the fore-most Heidegger scholars--and very accessible as well..Dreyfus Heidegger Lectures