Well, that makes it easier! Another possible source is Robert de Clari, another Western Crusader whose chapters from 18 on fall into what you are seeking available here. For secondary works you might see The Fall of Constantinople here. And, well, you might also read Umberto Eco's fictional Baudolino, which is set during the Fourth Crusade and features some hindsight on events previous to the fall. It is one of my favorite books. You can see if you like it by reading a limited preview here.It makes quite a good deal of fun especially of Niketas Choniates, whose Greek text if you ever read it will make your head explode. Well, happy reading and I am sure we await a great American novel. If you have further questions ask here or try the BYZANS-LIST of scholars and enthusiasts, which you can join by following the instructions here, though they will bite your head off if you do not fully explain your purposes!