Start with high-confidence existentialism books, then narrow by subgenre or adjacent topics.
The Present Age
Soren Kierkegaard, Søren Kierkegaard
Situating Existentialism
Jonathan Judaken, Robert Bernasconi
The Birth of Tragedy / Unpublished Basel Writings (winter 1869/70–fall 1873)
Friedrich Nietzsche, Alan D. Schrift
All Things Shining
Hubert Dreyfus, Sean Dorrance Kelly
Heidegger
Richard Polt
Existentialism
Thomas E. Wartenberg