There was no universal law that applied to assault but nobles did assault each other and there were lots of customs about how to deal with them. As usual for questions about the Middle Ages, I feel like I should always start off with “it depends on where and when”.

One popular way of dealing with it was trial by combat. There were complex rules for that though, at least if you made an accusation in court. First, if the assault took place in front of everyone, there was no need to prove it happened, so there was no need for a trial.

But if there were no witnesses, you could accuse someone of assaulting you, and you could fight the other person (or one or both of you could hire someone to fight on your behalf). You could also combat for other reasons, if you accused someone of stealing from you or wounding your honour in some other way, but the offense had to be severe enough to lead to combat - the stolen or damaged property had to be worth a certain amount of money, or the assault had to cause an evident wound, like an open cut or a broken bone. Otherwise the court would settle the case some other way, probably with a fine.

You had lots of opportunities to settle the dispute before the combat took place, because it could be extremely brutal and one or both of you could end up dead. Typically you had to wear specific equipment and meet at a specific place and swear specific oaths, so there was a religious aspect to it - priests were there to accept the oaths being sworn on the Gospels, and to consecrate the battlefield. By the 12th century everyone was starting to realize this was really not a good way to solve a problem, and in 1215, the church forbade priests from participating. That stopped legal combats a little bit, since the important religious component was removed, but it didn’t stop them entirely. Secular leaders also eventually followed the church’s lead and banned trial by combat as well.

But if you really wanted to fight an extra-judicial duel, go right ahead! Since duels continued at least until the 19th century, everyone apparently felt that defending their honour was an ancient right, regardless of whether it was illegal or not.

The same rules applied to nobles of any rank, since the status of being a knight in theory meant that everyone was equal no matter how rich or poor they were.

Things were different if you attacked a non-noble person, assuming that person was free (so a city-dwelling merchant, for example, or a free serf or peasant). A non-noble person couldn’t fight you because they were unequal in status, no matter how serious the injury or damage. The case would have to be settled by a fine, but you wouldn’t have to pay as much, maybe only half as much as you would pay to a fellow noble.

But what if you were the non-noble person and you're the one who attacked the knight? Well then you would have to fight! But you’d definitely lose. As a non-knight you don’t have the training and experience to defeat a knight in combat. So...don’t assault a knight, is your best course of action there.

For non-free peasants and serfs and slaves, they had very few rights, if any at all. You could attack your own unfree peasants, if you really wanted to. That would be very cruel, and you’d be harming your own economic base, but peasants had no legal recourse. There was no way to stop you, aside from other knights appealing to your own sense of dignity. Maybe you’ll stop if everyone thinks you're an asshole...maybe.

Assaulting someone else’s serfs in a different fief was a crime though, so that brings us back to the first example. A knight could accuse you of assaulting his serfs and causing damage to his property, which was one possible reason to fight a trial by combat against you.

Lastly, there were also laws against assault among free non-noble people. At least before duels were prohibited, they could participate in trials by combat as well, but only with wooden weapons, and no armour or horses (i.e. basically they would beat each other with sticks).

I don’t know if there is a specific book about the crime of assault among medieval knights. Since there was no universal law, the examples I gave are just a few of the many possible customs. For a basic starting point I would suggest:

Robert Bartlett, Trial by Fire and Water: The Medieval Judicial Ordeal (Oxford University Press, 1986)

He talks about the other kinds of ordeals (being dunked in water, carrying a hot piece of iron) but he does talk about combats and duels as well.