I have found Luther's lectures on Galatians to be absolutely riveting. Laid within his comments about the text of Galatians are golden nuggets of counsel clearly based on his own experience like what he says about defying Satan in times of despair concerning our sin-
"O cursed Satan, you choose a nice time to talk to me about doing and working when you know very well that I am in trouble over my sins. I will not listen to you. I will listen to Christ, who says that He came into the world to save sinners. This is the true Christ and there is none other. I can find plenty of examples for a holy life in Abraham, Isaiah, John the Baptist, Paul, and other saints. But they cannot forgive my sins. They cannot save me. They cannot procure for me everlasting life. Therefore I will not have you for my teacher, O Satan."