Joseph Conrad.
"Heart of Darkness" and "The Secret Sharer".
Introduction by Albert J. Guerard.
New York: Signet Classic / New American Library / Penguin Books USA Inc., 1983.
Since these stories are easily available in many editions, I will not provide links for the particular edition (now out-of-print) that I read.
Joseph Conrad, Wikipedia.
Heart of Darkness (1899), Wikipedia.
"The Secret Sharer" (1910), Wikipedia.