*rubs hands* eeexcelleent… Welcome to the madhouse
I highly recommend "Roman Military Equipment" by Bishop & Coulston. Still one of the best sources available. Connolly's "Greece and Rome At War" is a little outdated on some specifics, but still a very nice source and wonderful illustrations.
"The Complete Roman Army" by Adrian Goldsworthy is a good -general- reference, it can lead you to other sources and is a good "quick guide". Don't let the title throw you off, "The Complete" is a series of books.
"Aspects of Roman History 31BC - AD117" by Richard Alston, excellent overall look at all-things Roman, not just Military. Actually anything by Alston is excellent.
"Roman Military Dress" by Graham Sumner, there is also a 3-part series from Osprey Publishing ("Roman Military Clothing")
"The Roman Army: A Social Institutional History", Pat Southern
"Rome and the Barbarians 100BC - AD400", Osprey Publications - Ancient Society & History
All of these titles will have a lengthy works cited listing to look even more deeply at several sources, both Academic and "popular"
Also check Leg XX's pages,
www.larp.com/legioxx
Do Not get anything from Dando-Collins. Just, don't.
(While his work is all fun to read and all, his research and lack of academic sources leaves a lot to be desired, he also does some fanciful gap-filling)