Hello Damian!
The best starting point would still be:
H.R. Robinson, The Armour of Imperial Rome (London 1975)
But it is long out of print and if there's a used copy somewhere it has his price...
When you have the chance to lean a copy, do it!
Robinson's typology is today still used by English scholars, but it is somewhat inflexible because of Robinson's attempt to put every variant of a helmet type into a chronological development.
But as it pictures and describes nearly every Roman helmet known in 1975 it is still an important book.
A refined version of the continental helmet typology (for example Weisenau/Nijmegen instead of Imperial Gallic Type 'A')has been done by:
Markus Junkelmann, Römische Helme (Mainz 2000)
If you understand German, the following books are very useful:
H. Klumbach, Römische Helme aus Niedergermanien (Bonn 1974)
G. Waurick, Römische Helme, in: A. Bottini et al, Antike Helme (Mainz 1988
For 'sports' and 'Parade' helmets (and armour):
J. Garbsch, Römische Paraderüstungen (München 1978)
M. Junkelmann, Reiter wie Statuen aus Erz (Mainz 1996)
H. Born - M. Junkelmann, Römische Kampf- und Turnierrüstungen (Mainz 1997)
For late Roman Helmets:
H. Klumbach, Spätrömische Gardehelme (München 1973)
Ch. Miks, Vom Prunkstück zum Altmetall. Ein Depot spätrömischer Helmteile aus Koblenz (Mainz 2008)
M. Vogt, Spangenhelme (Mainz 2006)
If you read French:
M. Feugère, Casques antiques (Paris 1994)
And finally forthe Italic/Roman bronze helmets:
J.M. Paddock, The Bronze Italian Helmet (1993) which can be downloaded as a pdf file here:
British Library
These are the most important works.
There are of course papers on Roman helmets published in different periodicas.
Greets