An excellent place to start with Luther's own writings is The Large Catechism. It deals with some familiar stuff, but the depth is quite amazing. It was one of the first things I read in my years before seminary when I wanted to explore Lutheran theology.
And encourage reading The Augsburg Confession either in the Concordia: Reader's Edition or "straight" (I think there are some pamphlet size editions of the AC alone if investing in the entire BofC is too much to do at this time).
Another possible guide to getting into the Book of Concord and Lutheran teaching is the book Robert Jensen and Eric Gritsch co-authored about 30 years ago, Lutheranism.
And the collection of "essential" writings of Martin Luther that was done by Dillenberger might also be worth recommending. It isn't as extensive as that done by Timothy Lull, but it is still worthwhile.
I think the recommendations by others, especially the Forde, Braaten and Paulsen books, are also great selections for the list.