One great medieval war fiction book is 'The Pillars of the Earth' by Ken Follett. It vividly depicts the building of a cathedral during a time of war and political turmoil in medieval England. Another is 'Bernard Cornwell's 'The Last Kingdom' series. It follows the adventures of Uhtred, a Saxon nobleman, in the midst of the Viking invasions. Also, 'Game of Thrones' by George R.R. Martin, although it has a broader fantasy element, contains many elements of medieval - style warfare and power struggles among kingdoms.
A good one is 'Ivanhoe' by Sir Walter Scott. It is set during the Middle Ages and involves knights, battles, and the struggle between Saxons and Normans. Then there's 'The Once and Future King' by T.H. White. It retells the Arthurian legend with a focus on the chivalry and wars of that time. And don't forget 'The Name of the Rose' by Umberto Eco, which is a murder mystery set in a medieval monastery during a time of religious and political unrest, with elements of war in the background.
Well, 'The Song of Roland' is an excellent medieval war fiction. It is an epic poem that tells the story of Roland, a knight in Charlemagne's army. 'The Deerslayer' by James Fenimore Cooper also has elements of medieval - like warfare in the frontier setting. And 'Kristin Lavransdatter' by Sigrid Undset, which shows the life in medieval Norway, including battles and political unrest.