Rescuing the Gospel: The Story and Significance of the Reformation by Erwin W. Lutzer explores the history and importance of the Reformation. Readers who are unfamiliar with the Reformation will learn many new things, such as background information about Martin Luther, or about the Five Points of Calvinism (Total Depravity, Unconditional Election, Limited Atonement, Irresistible Grace, and Perseverance of the Saints). Some parts I found particularly interesting included the section on John Hus, as well as the chapter on Calvinism’s lasting influence. A nice touch of the book are the occasional pictures of historical artifacts and paintings. I think that the most important thing about the book is that it shows how the Reformation is relevant today.
You can find Rescuing the Gospel here.