One you should check out is Gerry Embleton and John Howe’s The Medieval Soldier.
Hey, thanks for the recommendation. Ordered it!
To the original poster, you said you have a few Ospreys... getting some more of them is always a good idea. In particular I'd recommend their Campaign series - each one has a really detailed look at one particular campaign or battle, and has some fantastic double-page battle scene artwork (the best by painter Graham Turner). For the 15th century there are some really good ones for the Wars Of The Roses (Towton 1461, Tewksebury 1471 and Bosworth 1485), plus The Fall Of English France 1449-1453 is great.
I know it's not perhaps what you were asking for (original 15th century sources), but if you are ever thinking of getting some more Osprey books, check them out.