Legal Systems Very Different from Ours by David Friedman is a book that does exactly was it says on the cover - describe a range of legal systems that differ substantially from the 21st Century Western oen we are familair with. There's a lot of world-building fodder in here. For example, in Saga-era Iceland crimes are hanlded in a simila rway to modern civil law. Imagine one of your PCs killing a barbarian in a bar fight only for the family to show up and sue you for the victim's blood price. Also the right to collect judgement was transferrable, so a group of weaker barbarians might offer to sell the party judgement rights for a powerful warlord at a discount, which is a great way to get a quest going.
Another interesting idea discussed is polycentric law. Tehre are four different schools of Sunni Islamic law and in many muslim societies all four were considered simultaneously valid. Imagine a cosmopolitan city that had different courts and legal codes dependign on who you were or what religion you followed.