(ed) J. L. Creed (Oxford, 1984) which has parallel Latin and English text.
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.