An Osprey book focusing on the men, tactics and equipment of the Roman army from its earliest days until Trajan's reign in the Imperial period. Be forewarned, despite the title, the book really is not about the legion or by implication of the use of "Caesar" the Empire.
As with all Osprey books, the writing is concise and the book is lavishly illustrated. However, given the heavy focus on describing the intricacies of swords, javelins, spears and armor, the book would have been well served with an initial description of the parts therein. Also, there are no diagrams of how a hoplite formation would have deployed in phalanx or how the manipular system was deployed in its trademark checkerboard fashion (though it is heavily, if somewhat confusingly discussed). This is one of the cases where a picture would have been worth a thousand words, even if it meant sacrificing one of the photos of a badly eroded grave stelae.
If you're interested in the place where archaelogy. literature and history met with regards to the Roman army, this might be a good starting place.