The “Further reading” section of Michael Reeves’s The Unquenchable Flame: Introducing the Reformation (Nottingham, England: IVP, 2009) flags six resources as “must reads” (pp. 189–91, numbering added):
1. Every Christian should read Roland Bainton’s classic biography of Luther, Here I Stand: A Life of Martin Luther (Abingdon, 1950). A rip-roaring bedtime page-turner.
2. And why not try reading some of Luther himself? You can find his great The Freedom of a Christian online . . . .
3. Calvin’s Institutes of the Christian Religion is a must. The title makes it sound scary; inside, it is easy to read and warm in style. If you can, get F. L. Battles’ two-volume translation of the 1559 edition (Westminster Press, 1960).
4. The first port of call [for the Puritans] has got to be The Bruised Reed by Richard Sibbes. Get your handkerchief out! Banner of Truth has made this easily available in their Puritan Paperbacks series. It is also online . . . .
6. The other excellent way into the benefits of Puritan wisdom is J. I. Packer’s Among God’s Giants: The Puritan Vision of the Christian Life (Kingsway, 1991). [A Quest for Godliness (repr., Wheaton: Crossway, 2010).]
Related: Mark Dever interviews Mike Reeves on the Reformation.