The best one I’ve read is the Traitor Son Cycle series by Miles Cameron. It’s fantasy, but it’s also has exceptionally realistic combat sequences, from arms and armour, to technique, tactics and strategy. The author REALLY knows his stuff when it comes to historical warfare. It doesn’t have any of the bad fantasy arms and armour tropes, since the author practises HEMA, wears historical armor, conducts tests on them and reads a lot of books about history. And even the world building itself is very much grounded in reality (unlike a lot of fantasy that is set in a weird pseudo-medieval-renaissance period, this is something Battle Brothers does very well too,) modelled after a specific time period in our own history with a touch of magic and wild beasts.
The rough time period it’s set in is very similar to Battle Brothers actually. Battle Brothers is basically late 13th/early 14th century, while the Traitor Son Cycle is set in late 14th century. It’s mostly about a company of mercenaries, each with very distinguished personalities, but there are some PoV of others such as royalty, nobles, knights, common soldiers, merchants and mages.
For non-fiction, a good one is John Hawkwood: An English Mercenary in Fourteenth-Century Italy.
