Like the title says. Especially if it incorporates players into the system well so they can use their abilities.
Ultimate Campaign has a Mass Combat System which I'll probably use later in my games. Convenient Link
I second the UCam mass combat rules, with an additional recommendation to get Ultimate Battle and Ultimate War from Legendary Games.
A good book is The Very Last Book About War, which I would peg at skirmish level: it uses a modified version of the normal combat rules to account for larger groups, and is theoretically scalable to truly massive armies, but it shines at smaller sizes.