Note, Feb. 16, 2022: I just made two slight edits here, to correct a typo and clarify a sentence.
16th-century German theologian Martin Luther, instigator of the Protestant Reformation, is a pivotal figure in Christian church history, in subsequent German history and culture, and in the history of the modern world as a whole. The larger picture in any of these areas can't really be grasped without understanding his influence. Having been raised in a Lutheran denomination originally founded by German immigrants, I naturally heard plenty about Luther as a child and teen in church (and parochial school for five years), though what I heard tended to be a hagiographic version of his life and a simplistic and uncritical distillation of his thought. I set about gratifying my curiosity for a fuller and more balanced treatment with this biography sometime after graduating from high school in 1970 (1971 is a rough guess as to when I read it). The book was a good choice for the purpose, usually considered to be the definitive Luther biography, and written by a liberal Quaker scholar who was basically sympathetic to his subject but able to view him with a certain detachment. Bainton was also an accomplished scholar (Titus Street Professor of Ecclesiastical History at Yale, an endowed chair also held by another church historian whose work made even more impression on me, Kenneth Scott Latourette), and literate in German.
This is a fairly thick book, with 386 pages of text; but it's narrative popular history for interested and intelligent lay readers, rather than very dry and pedantic academic history, and doesn't presuppose any specialized knowledge or vocabulary. It's a full-length biographical treatment, going back to Luther's childhood, as far as we know of it, carrying through to his death in 1546, and assessing his postmortem influence. The author doesn't employ footnotes, but he bases his text solidly on substantial acquaintance with primary and secondary sources, and documents the provenance of quotations (by page number and line) in a roughly 10-page list of references in the back. (As that indicates, there are quite a few quotations of Luther's own words, and those of his contemporaries.) Luther's theological and social thought, and that of the thinkers with whom he interacted (often in dispute), are presented in the context of his life, and in terms that make them intelligible to theological nonspecialists. Bainton doesn't whitewash his subject, giving an honest appraisal of the less palatable aspects of his career, particularly the savage controversial writings he produced in his pain-wracked and increasingly irascible old age.
By the time I read this, I had already embraced the concept of a church voluntarily composed of adult believers (or, at least, believers old enough to know in whom they're believing), and so had rejected the Constantinian conception (which Luther and many of the other Reformers retained) of a State-sponsored church entered by infant baptism. That conviction has only deepened in the decades since, and undergirds a strong belief that the greatest problem of the Christian church, which directly stems from that terrible historical wrong turn, is the popular identification of the "church" as a huge, amorphous mass of nominal "members" who have no more personal Christian beliefs than my daughter's pet house-cats (but who are thereby "entitled" to represent the church to the world and share in its governance). If the church had held to the believer's church concept in the second century and afterwards, IMO, or if more Christians had adopted it in the 16th, its subsequent history would have differed for the better. Closely related to this is my belief that the church's true unity is as the organic fellowship of believers united around a common loyalty to Jesus Christ as Lord and Savior, rather than as an adherence to a set of creedal formulations determined and enforced by a human authority (again, Luther didn't originate the latter view, but both he and his Catholic and Protestant opponents certainly embraced it with a vengeance). Bainton doesn't actually deal in great detail with any of these issues; but for me they tend to be inherent in the historical narrative itself, and to color my interpretation of Luther's significance. I think they dilute the practical impact of his insight about justification by faith (which was not as radical a break with historic Catholic thought as some modern interpreters treat it, though again Bainton doesn't discuss that in detail), and make much of his legacy an exercise in missed theological and ecclesiastical opportunity. (Some fellow Christians, whom I respect, would of course disagree with some or all of these views; but they do represent my honest principal take-aways from the book.)
Given the 1950 publication date, Bainton's 11-page bibliography of books and articles (which don't include Luther's own writings, though he made use of those too) is somewhat dated; but it's extensive enough to indicate a thorough familiarity with the entire scope of Luther scholarship up to that time. The book also has an over 12-page index, which is pretty comprehensive, and is enhanced by dozens of black-and-white illustrations, mostly contemporary woodcuts.
In summary, this is a must-read starting point for anyone who wants to seriously study Luther, the Reformation, or modern (post-medieval) Christian history. It stands as a worthy magnum opus for a distinguished historian.