Luther recommends...

These are books that Luther himself recommends your read:

'by 1537, when he wrote to Wolfgang Capito that along with his catechism the treatment of bound choice was his best work: "none of my works is worth anything except" the catechism and De servo arbitrio [The Bondage of the Will], he said.' (p. 15, Robert Kolb, Bound Choice, Election and Wittenberg Theological Method)

'Luther was extremely critical when it came to judging the value of his own books. The commentary on the Epistle to the Galatians (1531-35), the exposition of the Book of Deuteronomy (1523), and his sermons on four chapters of the Gospel of John (1528-29) he thought worth preserving because they were his only works really to contain theological teachings. He found the rest of his writings interesting solely from a historical point of view' (p. 168, Heiko Oberman, Luther: Man between God and the Devil).

'[Erasmus' arguments] have been refuted already so often by me, and beaten down and completely pulverized in Philip Melanchthon's Commonplaces [Loci Communes] - an unanswerable little book which in my judgment deserves not only to be immortalized but even canonized.' (p.102, Martin Luther, Introduction to The Bondage of the Will in Luther and Erasumus: Free Will and Salvation)

'It is a result of God's providence that the writings of Cato and Aesop have remained in the schools, for both are significant books. Cato contains the most useful sayings and precepts. Aesop contains the most delightful stories and descriptions. Moral teachings, if offered to young people, will contribute much to their edification. In short, next to the Bible, the writings of Cato and Aesop are in my opinion the best' (pp. 210–211, Martin Luther, "Table Talk Recorded by Anthony Lauterbach and Jerome Weller, 1536-1537" in Luther's Works, Volume 54: Table Talk, HT Tony Reinke)

These are books that Joachim Mörlin (1514-1571), the prominent early Lutheran theologian, and former student of Luther, suggested you read:

"Mörlin counseled beginning with the Small Catechism, for its three parts—law, gospel, and the table of responsibilities in the callings of daily life—summarize the entire Word of God. From the Small Catechism the reader should move to the Large Catechism and then to those works of Luther which summarize basic Christian teaching: the Schwabach Articles, the Confession Concerning Christ's Supper (1528), the Smalcald Articles, the Instruction for Visitors (1528), and the Augsburg Confession and its Apology, which Mörlin, like many of his contemporaries, counted among Luther's works as well as Melanchthon's. For learning the proper distinction of law and gospel the reader should turn to the Galatians commentary of 1535. Mörlin commended On the Last Words of David for its treatment of the 2nd article of the Creed, and On the Councils and the Church and Against Hanswurst for their treatment of the 3rd article. The Genesis commentary would summarize and complete the study of Luther's thought...

...The works of the earlier Luther, particularly the great trilogy of 1520—The Freedom of the Christian, The Babylonian Captivity of the Church, and The Open Letter to the German Nobility—were not on the lists, although they do appear on most such lists today."

(pp. 192-193, Robert Kolb, Martin Luther as Prophet, Teacher, and Hero, HT Benjamin Mayes)