Good places to start

• Internet Ancient History Sourcebook: Rome. Large number of Primary Source documents on Roman military history.
• Military Affairs of Ancient Rome & Roman Art of War. Concise entries on wide array of topics.
  
• Military of Ancient Rome. Better than average Wikipedia entry.
• Roman Army Bibliography. Lengthy bibliography of Secondary Sources.
• The Roman Army: A Bibliography. Lengthy bibliography of Secondary Sources.
  
• The Roman Military. Basic, useful information--a good introduction.
  

Books

• A Companion to the Roman Army, by Paul Erdkamp. Call # U35 .C648.
• The Complete Roman Army, by Adrian Keith Goldsworthy. Call # U35 .G646.

Article Databases

• JSTOR (Military History, Journal of Military History)
• Sage Complete (War in History)