formal classical education, Dürer was greatly interested in intellectual matters and learned much from his boyhood friend Willibald Pirckheimer, whom he no doubt consulted on the content of many of his images. He also derived great satisfaction from his friendships and correspondence with Erasmus and other scholars. Dürer succeeded in producing two books during his lifetime. "The Four Books on Measurement" were published at Nuremberg in 1525 and was the first book for adults on mathematics in German, as well as being cited later by Galileo and Kepler. The other, a work on city fortifications, was published in 1527. "The Four Books on Human Proportion" were published posthumously, shortly after his death in 1528. Dürer died in Nuremberg at the age of 56, leaving an estate valued at 6,874 florins – a considerable sum. He is buried in the Johannisfriedhof cemetery. His large house (purchased in 1509 from the heirs of the astronomer Bernhard Walther), where his workshop was located and where his widow lived until her death in 1539, remains a prominent Nuremberg landmark. Dürer and the Reformation Dürer's writings suggest that he may have been sympathetic to Luther's ideas, though it is unclear if he ever left the Catholic Church. Dürer wrote of his desire to draw Luther in his diary in 1520: "And God help me that I may go to Dr. Martin Luther; thus I intend to make a portrait of him with great care and engrave him on a copper plate to create a lasting memorial of the Christian man who helped me overcome so many difficulties." In a letter to Nicholas Kratzer in 1524, Dürer wrote, "because of our Christian faith we have to stand in scorn and danger, for we are reviled and called heretics". Most tellingly, Pirckheimer wrote in a letter to Johann Tscherte in 1530: "I confess that in the beginning I believed in Luther, like our Albert of blessed memory ... but as anyone can see, the situation has become worse." Dürer may even have contributed to the Nuremberg City Council's mandating Lutheran sermons and services in March 1525. Notably, Dürer had contacts with various reformers, such as Zwingli, Andreas Karlstadt, Melanchthon, Erasmus and Cornelius Grapheus from whom Dürer received Luther's Babylonian Captivity in 1520. Yet Erasmus and C. Grapheus are better said to be Catholic change agents. Also, from 1525, "the year that saw the peak and collapse of