Ammianus Marcellinus doesn't write about the 'Fall of the Roman Empire' as is seen in popular culture, but rather writes a history of the Empire from the accession of Nerva in 96CE to the Battle of Adrianople in 378CE. However, there are only eighteen surviving books that cover the period from 353 to 378CE at which point the Roman Empire (Both Western and Eastern parts) was still a force to be reckoned with. It is fortunate for historians that any of his work survives, because the fourth century has very few surviving texts, and there are large periods of time in which we have very little indication of what is actually going on, outside of one off references to a battle or the like.