If you are new to Roman military history, DO NOT read primary source literature. Roman literary sources are complex documents, which must be critically analyzed and contextualized with non-literary evidence; on their own, they do not provide an accurate introduction. As such, your best bet is modern scholarly literature. A great introductory book is Adrian Goldsworthy's Complete Roman Army. Goldsworthy is a top scholar, and this book is designed for a more general audience. It also has great illustrations and images, which help to visualize the army.