An Introduction to the book of Romans by Luther:
Romans is a glorious book. Luther introduces his thoughts on it. View Luther's introduction to Romans here.
Matin Luther's Commentary on Galatians:
If you're interesting in Martin Luther's commentary on Galatians you can read his thoughts on Galatians at this link.
Assorted Sermons by Martin Luther:
Various assorted sermons by Luther.
Luther's Bondage of the Will:
This is an influential work by Luther. It's a difficult read but has insight into Luther's views on the will. Read the Bondage of the Will here.
The North American Lutheran Church:
Visit the NALC website here - www.TheNALC.org