In this specific case? Osprey. I wouldn't use an Osprey book as a serious source either here or in a paper, but for your purposes, there's no beating them. Not only do they come with maps and tactical breakdowns so that you can see how the battle progressed, they also come with nice colour plates for visual inspiration. Their actual historical merit depends on the author, so check the name before you trust too much. To start you off and to contrast against the Romans, I'd recommend both volumes of David Nicolle's European Medieval Tactics (part 1, part 2), then browse their site for anything that strikes your fancy. If they happen to be beyond your reach...well, there are ways.

For more free stuff of a more academic bent, De Re Militari has a whole mess of it. Grab anything that catches your eye.

On the Roman end, anything by Adrian Goldsworthy is an excellent place to start from. His The Complete Roman Army is a pretty good overview of the, let's call it the Rome: Total War eras, from the mid Republic to the early Empire. You can then pillage the rest of his author page and his bibliography from there.

There is also our booklist, which is again a more academic bent, but will furnish you with excellent titles. If they happen to be beyond your reach...there are ways.

Good luck from a fellow fiction writer, and remember always, pavises rule, longbows drool.

Hi there - we're happy to approve your question related to your creative project, and we are happy for people to answer. However, we should warn you that many flairs have become reluctant to answer questions for aspiring novelists and the like, based on past experience: some people working on creative projects have a tendency to try to pump historians for trivia while ignoring the bigger points they were making, while others have a tendency to argue with historians when the historical reality does not line up with what's needed for a particular scene or characterization. Please respect the answers of people who have generously given you their time, even if it's not always what you want to hear.

Additionally, as amazing as our flair panel is, we should also point out that /r/AskHistorians is not a professional historical consultation service. If you're asking a question here because you need vital research for a future commercial product such as a historical novel, you may be better off engaging a historical consultant at a fair hourly rate to answer these questions for you. We don't know what the going rate for consultancy work would be in your locality, but it may be worth looking into that if you have in-depth or highly plot-reliant questions for this project. Some /r/AskHistorians flairs could be receptive to working as a consultant in this way. However, if you wish for a flair here to do this work for you, you will need to organize this with them yourselves.

For more general advice about doing research to inform a creative project, please check out our Monday Methods post on the subject.