Martin Luther considered "The Bondage of the Will" (In latin: "De Servo Arbitrio" 1525 AD) his best work.  You may read it online for free at CCEL. I read the new translation by J.I. Packer and O.R. Johnson, that's about 320pages and has a lengthy introduction about the book, Luther and Desiderius Erasmus (c. 1466-1536). 

Martin Luther was an Augustinian Monk, and this work is a honor to his master Augustine. Bondage of the Will is Luther's response to Erasmus's short book "Diatribe", where Erasmus attack's Luther's doctrine of Double Predestination. So Luther's book is in response to Erasmus' book, and Erasmus's book is in response to Luther's writings. In general, books in this format are not very fun to read and require too much of a back story. However, this particular work is on every Luther reading list I've seen for the great arguments it contains.

By: Wyatt Houtz