Axum -- Fact and Fiction

I recently read a very nice alternate-history/science fiction series (The Belisarius Series, which I highly recommend, by Eric Flint and David Drake) in which the civilization of Axum featured strongly. It was characterized as a society with a regimental army skilled in spear use and heavy hacking swords, but more of a naval and trading power than a military one, with ships that were (in general) relatively sleek and fast. It was mentioned that it fought quite a few battles with nearby seminomadic Arabian societies, and that it was a nominal ally of Byzantine Rome.

I was mostly wondering how much of this is accurate (Eric Flint is generally pretty good about historical accuracy, especially since he has a degree in African history), and if there are any good books about the progression of Axum as a civilization.