Glad to give you the name of what I have or recommend:
1. Imperial Governor-George Shipway, a novel of the Legions in Britain at the time of Boudicca's rebellion
2. Late Roman Infantryman-(www.ospreypublications.com
) Hereinafter Osprey
3. Roman Legionary-Osprey
4.Warriors of Rome-MIchael Simkins (good luck re; that one)
5. "The Roman Way of War"-in "The Great Battles of Antiquity" (expsensive)
6. The Roman Imperial Army of the first and Second Centuries AD
7 Republican Roman Army-Osprey
8. Roman Battle Tactics 109BC AD313
Generally a legionary in the Late Republic to mid imperial time signed up for twenty years. The basic unit was sometimes called the "Conturbiam" after the 7 man leather tent they used. Usually in these times prior to the reorganization of the Army By Constantine and subsequent to the Marian reforms each Legione could be considered to be made up of several layers of twenty year men recruited from an assigned recruiting area.
some were near the end ie: in the last 1/4 of their enlistment and then there were the new men just starting their 20 year terms.
Severus (I think) or Germanicus rduced the minimum time to 16 years, but the reform was Quietly forgottlen.
For later Empire Army Life :
Gods and Legions-Pressfield
Kind of General-"Roman Warfare" Adrian Goldsworth.
Osprey has many many books that will catch your eye.
The series of books introduceced to me Hugh written by Dando Stevens, now covering the Xth Legio Fretensis (Caesar's Legions), 6th Gallica, XIV Gemina (Nero's Killing Machine) (there are at least two others) are very much fun.
The Antagonists by Enrest K. Gann Re: Masada is great, as is its sequel "the Triumph" but they will be hard to find.
should keep you busy.