Lactantius, Divine institutes, (ed) Anthony Bowen and Peter Garnsey (Liverpool, 2003) which has original text, translation and commentary.
For Zosimus's New History see:
Zosimus, Historia Nova, The Decline of Rome (San Antonio, 1967)
VI FALL
The most up-to-date, accessible and authoritative history of Rome's decline is:
Heather, Peter, The Fall of the Roman Empire (London, 2005)
Other key works are:
Heather, Peter: Goths and Romans 332-489 (Oxford, 1991)
Heather, Peter, The Goths (Oxford, 1996)
Matthews, John, Western Aristocracies and Imperial Court, AD 364-425 (Oxford, 1975)
Ward-Perkins, Bryan, The Fall of Rome and the End of Civilization (Oxford, 2005)
Cambridge Ancient History, Volume 13: The Late Empire, AD 337-425', (ed) Averil Cameron and Peter Garnsey (Cambridge, 1997)
ANCIENT SOURCES
For Ammianus Marcellinus's history see:
Ammianus Marcellinus, The Later Roman Empire: AD 354-378 (London, 1986)
For Zosimus's New History see:
Zosimus, Historia Nova, The Decline of Rome (San Antonio, 1967)
For the fragments of Olympiodorus' Histories see:
Blockley, R C. (ed) The Fragmentary Classicising Historians of the Later Roman Empire: Eunapius, Olympiodorus, Priscus and Makhus, Volume 2 (Liverpool, 1983) which 1 Greek text, translation and notes.