I personally liked
“THE ROOTS OF THE REFORMATION” by Karl Adam.
It is a really good book. Does a good job of explaining why Luther broke away, said what he did and did not believe and gave some justification of what he was precisely rebelling/protesting against. It also didn’t pull any punches on what the problems in the Church were at the time. I highly recommend it!
The book concludes (rightly or wrongly) that Luther would have been fine with the Church after…hmmm…I think the first Vatican council…someone could/should correct me on that if I’m wrong (wrong about what the book says, not their understanding of Luther …
John