Slitherine

Here are couple more topical book recommendations:

Ian Heath's books (originally published by Wargame Research Group) cover the armies and "units" of their respective regions and periods with a good amount of detail and with plenty of illustrations. If you are interested in more detailed description of the structure and equipment of the armies of middle ages than what you typically get from the history books, these are great reads. They don't offer a particularly good overview of the surrounding historical events so without any prior knowledge of the period they can be a bit hard to follow.

The books had been out of print since the 1980's and 1990's but in last few years three of them have been reprinted and still available new.

Ian Heath
Armies of the Dark Ages 600-1066 (Reprinted 2015) -- Great companion book for the upcoming FoG2 Dark Ages DLC
Armies of the Armies of Feudal Europe 1066-1300 (Reprinted 2016)
Armies and Enemies of the Crusades 1096-1291 (Reprinted 2019)
Armies of the Middle Ages, volume 1 1300-1487 (Currently not available new) -- Covers Western Europe
Armies of the Middle Ages, volume 2 1300-1500 (Currently not available new) -- Covers Eastern Europe and Near East

Here's an example of the illustrations

I found Legion vs Phalanx by Myke Cole very handy for understanding some of the intended mechanics in FOG. The two pair up very well in my opinion.