Some thinkers posit that there are five primary existential philosophers: Jean-Paul Sartre, Friedrich Nietzsche, Martin Heidegger, Karl Jaspers, and Gabriel Marcel. Their books explore different aspects of existentialism, and offer different perspectives on this unique philosophical movement.

Jean-Paul Sartre was a French philosopher who is considered to be the founder of existentialism. His book, “Existentialism is a Humanism”, explores the central concepts of existentialism, and argues that existentialists are humanists because they believe in the inherent potential of humans to create their own meaning in life.

Friedrich Nietzsche was a German philosopher who was highly influential in the development of existentialism. His book, “The Birth of Tragedy”, explores the concept of the Dionysian, which refers to the power of the creative artist to confront the meaninglessness of life and to affirm life in spite of it.

Martin Heidegger was a German philosopher who was highly influential in the development of existentialism. His book, “Being and Time”, is considered to be one of the most important existentialist texts. It explores the concept of Da-sein, which refers to being-in-the-world. Heidegger argues that Da-sein is always situated within its context, and that it is impossible to divorce oneself from one’s surroundings.

Karl Jaspers was a German philosopher who was highly influential in the development of existentialism. His book, “Philosophy: An Introduction”, provides an overview of existentialist thought, and discusses how existentialists approach philosophical problems.

Gabriel Marcel was a French philosopher who was highly influenced by existentialism. His book, “The Mystery of Being”, discusses different aspects of existential thought, including freedom, being, time, and others.