"The past is a foreign country: they do things differently there."
- L.P. Hartley
Thanks to Black Gate, I discovered these:
Tales from Froissart
They're translated from French - the common language of the nobility at the time, as far as I understand it. They're not complete, but the snippets are interesting.
Reading Medieval history can be entertaining for its own sake. But it's also useful for gaming. It's a good reminder that it's not just a lower-technology present. It's socially different on a scale that can be difficult to grasp. They'll use the same words we do ("ally" or "King" or "God's Will") and have a very different meaning. Ally - does it mean friend or enemy of your enemy? How powerful is a King (absolute rulers were a thing of the further past and closer present, not of the time)? Is "God's Will" they were saying with real meaning and not just as a handwavy explanation of why they won or lost?
How they act is equally interesting.
Do you run to help your ostensible ally in a battle against your foe, but in order to do so risk getting less credit than he for the victory? Sometimes the answer is no.
Do you win the battle but then surrender to your foes? Sometimes yes, because your foe's allies might be more worrisome than the guys you just whupped.
A King helping his fellow King out of the rightness of the cause - does he also expect to get paid for the help? Sometimes yes - seems like always yes.
I consciously game in a modern-informed faux-medieval world. More Renaissance than Medieval, honestly, because I find more connection to the dawning modern world. Players, too.
If you're beaten in a fight and your foe claims your weapons, armor, horse, and asks for a ransom, do you cheerfully pay it and think, "Next time by the Grace of God it'll be my victory!"? Or do you write his name down on your character sheet and think, "I'll get my stuff back and double-murder his corpse!"? If the latter, you're probably assuming a more modern mindset for the game.
If you assume war is groups marching around seeking to force battle or cities as prizes, with social disorder rising up as the war grinds on, armies raised and falling as money and season permits, you're probably in line with these tales. If you assume a war has two sides, front lines, and orderly safe rear areas behind organized and orderly professional armies, you're probably assuming a more modern world.
That's just a couple of examples - read enough of the Tales from Froissart and you'll find plenty of places where you'd probably act differently as a PC than they are said to have acted in reality. It's a different world now. But if you're going to have some medieval in your faux-medieval, this is a readable and quickly readable source for some of what they valued enough to write down.