First of all, I would recommend the “Name of the Rose” from the great medievalist Umberto Eco. The book is a fascinating peek not into the details of medieval life and customs, but also into the people’s minds and the way they thought. It’s not exactly light reading with numerous Latin quotes and references to contemporary literature and events, but it’s very authentic and informative.
Sapkovsky, the auther of the now famous “Witcher” saga wrote a trilogy about the Hussite uprising in the lands of the Czech and Polish crown - the books are called “Narrentum”, “God’s Warriors”, and “Lux Perpetua”. These books, while riddled with fantasy elements like ghouls, spectres and witches, together with authors weirdly eroticised concept of magic, still represent a solid reading and the author has certainly researched the topic well. The writing is original, peculiar, and the books read very well, often introducing a surprise view on actual historical events or dilemmas.
Lastly, I would recommend “Teutonic Knights” from the Polish author Henryk Sienkiewicz dealing with the struggle of Poland with their arch-enemy - the Order of Teutonic Knights.