Just like my child will probably laugh at the concept of an iPad someday, there are parts of The Albigenses that don't resonate well with a 2010 audience. The first half or so of the fourth volume is a very detailed battle scene. There is quite a bit of untranslated Latin and French throughout the book. And the mere fact that it's four volumes means that it's a big commitment for a reader.
Which is too bad, because this is a really great story. The characters are interesting, reading about their adventures was really cool, and I managed to learn something about a Crusade I had no idea existed. There are really unexpected plot twists and I liked the medieval setting. I'd love to somehow see this book updated for a modern audience but I don't think that is usually done. And as Mike pointed out, literature professors would probably be horrified at the suggestion.