Stephanos, your question is fairly broad. For the siege of Constantinople in 1204 by the Western Crusaders and their reactions on reaching one of the richest cities in the world, I recommend Geoffry de Villehardouin here. There is also a limited preview (some of the pages have been cut out) through Google Books of Nicetas Choniates here, though the translation is rather dorky and awkward. There is also the account of Benjamin of Tudela, which falls roughly in the period about which you are inquiring here. I hope these help if you have not already found them.