Unlike other armies in antiquity, the Roman army evolved to be a formal institution with a distinctive military code, standard equipment, defined ranks and duties, as well as laws and procedures affecting the life and retirement of its soldiers. Although service was long (20 years/no family allowed) and discipline was strict (i.e. decimation), it was truly the first modern professional army with very specialized units ranging from doctors and cooks to sappers and siege engineers. Roman soldiers usually came from the poorer elements of society by the time of the Late Republic and received a thorough training as primarily infantry men: cavalry being left either to the equestrian class or mostly foreign auxilliaries. Their training, efficiency and tenacity allowed the Roman Legions to fight cohesively as flexible units to overcome superior numbers under higher attrition. It made Rome the master of the Mediterranean world and most of modern Europe for over 1500 years (counting the Byzantine.)
G.R. Watson provides a historical and sociological analysis of the Roman army from the perspective of the individual soldier as opposed to being just a general chronological summary or studying it more under a political analysis. Watson covers the subjects in the order that a new recruit would encounter them such as sign up, training, terms of service, etc. The book doesn't study the evolution of the Roman army per se as he uses the later Roman Imperial army as a model as it had pretty much fully evolved in its institutions by that time. Watson reinforces his description and summary with various examples from different periods in the Roman Empire without resorting to a chronological order.
This is overall a good book that gives the reader an idea as to what the Roman army was like from the perspective of the average soldier. Despite being a thin paperback, the book's conclusions are supported by extensive and thorough historical references and is easy to read for any level of education. I strongly recommend this book and Adrian Goldsworthy's "Roman Warfare" for a good textual foundation in learning more about the Roman army.