For anyone running a game using early firearms, I highly recommend this book:
It's a highly readable and well-detailed book on the development of warfare as gunpowder took on a greater and greater role.
It's a useful book for exploding some pernicious myths. Such as, say, "guns came along and armor went away because it was useless" (yeah, so where did musket-proofed plate come from? And why did Napoleon's army including heavy cavalry wearing armor?) And the idea that knighthood ended when guns came along - it was the nobility that purchased these guns, to cement their power.
The details of the battles and campaigns are excellent, too.
It's well worth reading if your fantasy game of choice includes some gunfire.