Just got done reading a pretty good treatment of the 4th crusade by Jonathan Phillips. Have you seen that one?
I haven't read it but it sounds excellent. I just added it to my list. Many years ago I had quite a fascination with the 4th Crusade, and read everything I could find on it. I consider it one of the more dreadful tragedies of history not only for the geopolitical consequences (plunging the Byzantine Empire into the decline that eventually permitted the Ottoman conquest) but it also swept away much of the cultural heritage that had been preserved of ancient Greece and Rome.