I just finished a book by Stephen Dando-Collins called "Caesars Legion", which follows the Tenth from it's very first enlistment in Spain by Julius Caesar, through his Iberian and Gallic campaigns, civil wars, and service in the East for hundreds of years. It contains a lot of details that I wasn't aware of, such as legions not being "topped up" with new recruits between enlistment years (16 years initially, increased to 20 by Augustus). Using this "enlistment factor" they can determine the average age and experience of a legion at various times throughout history.

It's a fairly interesting read for a non-fiction book, and not especially "heavy", so easy to follow. I'd recommend it for anyone following this thread wanting to find a little more detail, without wanting to have to read a scholarly text.