I know you said Medieval, but if you like war novels then you may want to check out the "Gates of Fire" by Steven Pressfield. It is set in the Hellenic period at the time of the Persian war. The book is very historical as most references came directly from Herodotus and other historians of the time. It is an amazing story and extremely popular even ten years after it was written. It is very gritty, real, and informative of life at the time. It can be tough to find on the shelves as it is still in high demand, but it is well worth the look. The Gates of Fire is on the Commandant of the Marine Corps' reading list. It is taught at West Point, Annapolis, and at the Marine Corps Basic School at Quantico at Virginia Military Institute.
Go tell the Spartans, stranger passing by, That here, obedient to their laws, we lie. Nearly 2,500 years ago, in 480BC, at a bleak pass in a far-flung corner of eastern Greece, three hundred Spartan warriors faced the army of Xerxes of Persia, a massive force rumoured to be over a million strong. Their orders were simple: to delay the enemy for as long as possible while the main Greek armies mobilized.For six days the Spartans held the invaders at bay. In the final hours - their shields broken, swords and spears shattered - they fought with their bare hands before being overwhelmed. It was battle that would become synonymous with extraordinary courage, heroism and self-sacrifice: it was Thermopylae. In Gates of Fire, Steven Pressfield tells the epic story of those legendary Spartans: the men and women who helped shaped our history and have themselves become as immortal as their gods.