Martin Luther 
Commentary on the Epistle to the Galatians [PDF ebook] 

A classic commentary on Galatians by one of the world's greatest expositors.
After reading the Bible, every Christian should at least one time in their life read something by Martin Luther to understand the man who has been responsible for the Protestant Reformation and the issue of justification that was at stake.
Historically, this is one of the most important commentaries ever written. It embodies the reformation teaching of justification by faith alone. Galatians was Luther's favorite epistle, and his commentary on Galatians was his favorite among his own writings.