I keep my book review ratings simple—they’re either required, recommended, recommended with qualifications, or not recommended. If you want the TL;DR, this is it:
Recommended: Thomas Kidd’s What is an Evangelical? is a short and accessible history of “evangelicalism”: where it came from and how it developed into the multifaceted movement it is today, including but very much not limited to its political engagement. Even-handed and well-researched, but accessible – you don’t have to be a religious history buff to follow this (and if you are it’ll probably be a lot you already know). The book’s biggest weakness, sadly, is Kidd’s prose: it gets the job done, but gets pretty monotonous. That aside, this is a great introductory history of the movement that is such a cause of political consternation.