Without listening to the podcast, my assumption would be he was making the distinction between the Christ of history and the Christ of Kerygma. He is correct in the sense that Historical studies have limitations, and historical Jesus studies tend to do best when they are held together with the Christ of Kerygma. That being said I would say that Ehrman's book is fairly introductory so i don't know that you would really need any pre-reading. If you're looking for more historical Jesus studies John Meier has a multi-volume work called "a marginal Jew" that is pretty extensive (though it seems to be controversial in some circles) or Richard Horsley's "God and Empire" which is much shorter and does a great job of placing Jesus in his historical context, or even Marcus Borg's "meeting Jesus again for the first time." None of these deal with the historical development of Christology, but they are all great examples of how historical Jesus studies look at the Jesus of history. If you're looking for something more systematic, then any of the modern books I recommended would be great (with the caveat that Pannenberg can be really difficult to read). If you're really interested in the methodolgies this book looks like it will be helpful (I admit I haven't read this book, it is just on my ever growing list of books to read) but it looks like it might be really really dry, so you'll probably want a good wine to go along with it. Hopefully that helps.