Being and Nothingness
"The Sickness Unto Death" has strong existentialist themes. For example, the concepts of the finite and infinite parts of the human self translate to Heidegger's concept of 'facticity' and Sartre's concept of 'transcendence' in "Being and Nothingness". Kierkegaard's thesis is, of course, in other ways profoundly different from Sartre, most obviously because of Kierkegaard's belief that only religious faith can save the soul from despair. This particular brand of existentialism is often called Christian existentialism.