Fundamental Principles of the Metaphysics of Morals

Fundamental Principles of the Metaphysics of Morals by Immanuel Kant, translated by Thomas Kingsmill Abbott

Kant can be dense. He explores metaphysics and morals. Where do morals come from? What about animals? Can they be moral? The discussion can be quite difficult to follow.

This was originally Grundlegung zur Metaphysik der Sitten in German, and has some alternate English translations, such as Foundations of the Metaphysics of Morals, Grounding of the Metaphysics of Morals, and the Grounding for the Metaphysics of Morals.

An Answer to the Question: What is Enlightenment

An Answer to the Question: What is Enlightenment by Immanuel Kant

In this very short essay, Kant defines enlightenment. To him it is the freedom and the willingness to challenge one's thoughts. It is the emergence from one's immaturity and the willing to think for oneself as opposed to having others tell you what should be thought. 

I listened to the version from librivox which was from a translated source. However, I cannot seem to find who the original translator was.

Kant's Prolegomena and Metaphysical Foundations of Natural Science

Kant's Prolegomena and Metaphysical Foundations of Natural Science by Immanuel Kant, translated by Ernest Belfort Bax

This book includes a discussion of Kant's life and career and then two of his shorter works.

Prolegomena to Any Future Metaphysics That Will Be Able to Present Itself as a Science (1783)

A brief introduction to metaphysics and defense of Kant's writings. This is written as more of a discussion of philosophy and Kant's thought and metaphysics.

Metaphysical Foundations of Natural Science (1786)

This shorter work reads more as a math and science book. There are discussions about gravity and other similar physical phenomena and how these are built on metaphysics