The non-Lutherans of the Reformation, such as John Bunyan, were more Lutheran than the Lutherans of today, because they read Luther and we do not. Luther demands an intellectual effort, but those who expand their capacity for the Word through Luther will never accept anything less. Walther taught: "The closer to Luther, the better the theologian."
Everyone should make a regular study of the Small Catechism, the Large Catechism, and the Smalcald Articles, all in the Book of Concord.
A beginner would do well to start with Luther's Family Devotions, from Mark V Publications. Veteran readers of Luther also enjoy this wonderful collection of the Reformer's thoughts, divided into segments for each day of the Church Year, each day ending with a selection from The Lutheran Hymnal.
Two sets of Luther's sermons, the church postils and the house postils, can be ordered through Barnes and Noble.
Another source is Christian Book Distributors, where the following great Luther books can be ordered:
Day by Day We Magnify Thee
The Galatians Commentary
1 and 2 Peter and Jude Commentary
The Large Catechism
Ewald Plass edited What Luther Says, published by Concordia Publishing House, and wrote a fine biography of the Reformer, This Is Luther, now out of print.
Northwestern Publishing House (WELS) is another source for Luther books.
Many Luther books can be bought at significant discounts from seminary bookstores. Also, look for them at book sales or used book stores.