What is a good, authoritative source on medieval government?
I'd like to know more about this topic as someone mentioned it to me today. I'm looking for information on how said governments worked, how they rose and fell, how they were connected/influenced each other etc. I don't exactly know much about the period so I'd preferably like a suggestion that has "pleb" terminology so I don't have to go out of my way to research topics related to what is said. I also don't exactly have money to splash around at the moment so I'd prefer if it's a free source.
In other words, what is a good website/book for me to read to gain a better understanding of governments during the middle ages?