New techniques and new document finds have completely re-written what we thought we knew about the history of Christianity. It turns out that the Christian ecosystem was much richer and more diverse than most people understand. The variety of different beliefs held among people who called themselves "Christian" is staggering. Fortunately, new documentary evidence illuminates what these people believed, and what sorts of books they considered to be scripture.
Ehrman also explains the history of the more traditional books in the canon. The New Testament takes on additional life once you understand the history behind each of the books.