Check out the Alexiad, a primary source written by Byzantine princess Anna Komnene (regarded as the first female historian - and a brilliant one at that!), which describes the life of her father, Alexios I Komnenos, and the extraordinary things he did to save the Byzantine Empire from certain death. It also describes the events of the First Crusade from a Byzantine perspective, painting a more complete picture of the Crusades and the complex relationship between the East and West.

It's probably my favorite book of all time, and I highly recommend it to anyone who doesn't know much about the significant role Byzantium played in many events occurring during the Medieval period in Europe. It also acts as a great introduction to the Byzantine world and it will probably blow your mind to learn about how much these people knew almost a thousand years ago.

You might also want to check out "The Plantagenets: the Kings who made England" by Dan Jones

Also, "The Crusades: The Authoritative History of the War for the Holy Land" by Thomas Asbridge

One of my all time favorites is

Kantorowicz, Ernst Hartwig. The King’s Two Bodies: A Study in Mediaeval Political Theology. Princeton Paperbacks. Princeton, N.J: Princeton University Press, 1997.

As the title suggest, it is about the linking of the king and the divine, the person and the office, in England and France