If you come into this book with a good knowledge of the Bible but a fairly vague knowledge of other ancient Christian works, as I did, you're in for a mind-bending treat.
Ehrman picks a number of "Lost Scriptures" -- that is, books which were at one time considered sacred or near-sacred Christian works but have, for various reasons, not been included in the current Bible -- and he gives a brief prelude to each before offering their English translations. He breaks these books up into 5 groups: the Lost Gospels (think Gospels), Acts (think Acts), Letters (think Paul's Epistles), Apocalypses (think Revelations), and Sacred Cannons. The last section is merely a sample of some lists of what ancient Christians considered sacred books.
What this book deals with is primarily the source documents. That is to say, assigning context to said documents is not this book's mission. Instead, it tries to give a survey of what we now call lost Scriptures.
Confoundingly, many of the books are only published in fragmentary form. In many cases, this was not optional because of the fact that only small fragments of the source documents exist; in the astounding Gospel of Peter, for example, we have only what appear to be the last few chapters, beginning with Pilate at the trial. While this was usually not Ehrman's fault, it was rather frustrating at other times when he truncated some of the books himself, presumably in the interest of saving space.
I read this book in tandem with Ehrman's "Lost Christianities," and I highly recommend doing so. "Lost Christianities" provides historical context for the raw materials of "Lost Scriptures." Brace yourself before beginning, however, because both books are dense and demand considerable attention to detail.
If you are already versed in this genre, I'm not really sure that Ehrman intended these books for you. He essentially writes this book as a source book.
On a personal level, the number of references to Mary Magdalene in these works, the varieties of Christianities that they represent which are totally foreign to us, and the general fuzziness between the denouncement of books as forgeries or heresies versus thier acceptance divinity was eye-opening. These books ran the gammit from agreeing with the New Testament to disagreeing with it to being too crypitc to decipher to being... well, downright creepy. The measures that were taken by groups in history to ensure that some of these works would remain hidden is also disturbing.
To conclude, if you're looking for a general primer into the nuts and bolts of lost Christian writings, this is as good of a place as any to begin.