https://archive.org/details/historyofthemiddleages/
The problem is that most Marxist historians that concern themselves with feudalism and the Middle Ages write country-specific. I've yet to encounter a comprehensive Marxist book about the Middle Ages outside of textbooks like this, if you can read German Ferdinand Seibt's Glanz und Elend des Mittelalters is a good introduction but it's not Marxist. Most medievalists dislike Marxists because they tend to use historical categories, which, according to the bourgeois historians, can be applied to capitalism as it is a more unitary system but are not nuanced enough to describe feudalism. See this critique:
https://www.versobooks.com/blogs/4105-marxism-and-the-middle-ages