I've long wanted to do some player handouts or booklets specific to each class. Someone wants to play a fighter, give them a handout with all they need to know about fighters: how shields work, weapon damage, missile range, experience chart for leveling. I haven't made these yet.
But I was just thinking that in addition to giving that player everything they need to know at their fingertips, it could be quite nice for more casual players to have rule expertise in one realm and not need to worry about the rest.
So, if you have a house rule for armor wear and tear, let the fighter know about it, and they can explain it to the other players. If you have special resurrection, level draining, or healing rules, let the cleric/priests become the expert on that. This gives each player a reason to be important and it would probably increase the number of house rules/mini-games you could get away with without becoming too complex, because no players will have to know them all.
Traditionally, a lot of the rules can be dealt with when need arises and only the DM needs to know the rules at all. But when your start talking about subsystems like hunting, repair, special house rules dealing with travel, players need to know this stuff to be able to make decisions. So, split up the responsibility of that rule knowledge.
Magic users could know about your world's languages and writing systems, all about scrolls and making them, perhaps about maps and curses and such. Maybe also enough vague history of your realm to know what things are older than others.
I would probably include a lot about the undead for divine petitioners, and like I mentioned above, how healing and disease works.
Thieves would know any relevant lock mini-games, of course, but would be a good place to put knowledge of the values of treasure sold in different towns, the going rates for different gems and different types of coins, etc.