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.