Here I Stand: A Life of Martin Luther by Roland Bainton. This is a classic book on Martin Luther, and one I read several years ago. https://www.amazon.com/Here-Stand-Life-Martin-Luther-ebook/dp/B00E80Q6X8/ref=sr_1_1_twi_kin_1?ie=UTF8&qid=1504694469&sr=8-1&keywords=Here+I+stand