Four of Martin Luther’s fundamental works can be found in Basic Luther.  These works include The Ninety-five Theses, the document which sparked the Reformation; Address to the Nobility, which asserted the spiritual independence of the laity and attacked church corruption; Concerning Christian Liberty, a statement of Luther’s basic theological position and an exposition of the doctrine of justification by faith; and A Small Catechism, an instruction in the elements of Christian belief and practice.  See section 284 “Lutheran Church/Luther’s Writings.”

From Theopedia.com on the significance of the 95 Theses:

The 95 Theses, a document written by Martin Luther in 1517, challenged the teachings of the Catholic Church on the nature of penance, the authority of the pope and the usefulness of indulgences. It sparked a theological debate that fueled the Reformation and subsequently resulted in the birth of Protestantism and the Lutheran,Reformed, and Anabaptist traditions within Christianity.

Luther’s action was in great part a response to the selling of indulgences by Johann Tetzel, a Dominican priest, commissioned by the Archbishop of Mainz and Pope Leo X. The purpose of this fundraising campaign was to finance the building of St. Peter’s Basilica in Rome. Even though Luther’s prince, Frederick the Wise, and the prince of the neighboring territory, George, Duke of Saxony, forbade the sale in their lands, Luther’s parishioners traveled to purchase them. When these people came to confession, they presented the plenary indulgence, claiming they no longer had to repent of their sins, since the document promised to forgive all their sins.

Luther is said to have posted the 95 Theses on the door of the Castle Church in Wittenberg, Germany, on October 31, 1517. Church doors functioned very much as bulletin boards function on a twenty-first century college campus. The 95 Theses were quickly translated into German, widely copied and printed. Within two weeks they had spread throughout Germany, and within two months throughout Europe. This was one of the first events in history that was profoundly affected by the printing press, which made the distribution of documents and ideas easier and more wide-spread.