boleslaws crown , boleslaws sword szczerbiec [for the later start date]
maby armour or weapons associated with the 3 slavic brothers lech czech and rus

More books you could implement are the Tao Te Ching, and the Mahabarata (which would be hilarious if it was just the size of an entire wall, story is friggin massive.) I imagine the ones you mostly suggested as well as these two would be fairly common (keeping in mind that books in general were fairly UNcommon) so it would make sense to keep their value somewhat low. In contrast if you were to find the Nag Hammadi Library texts or a text saved from the Library of Alexandria they might be worth more. The Cyranides or Book of Thoth or the Emerald Tablet would be cool, though I imagine most people wouldn't have a clue what those are (though maybe it would lead to them googling it, and learning is ALWAYS good!)

I know you're trying to keep it simple, but it occurs to me that it'd be cool if after you obtain a certain number of books you got a decision that allowed you build a nifty bookshelf to house them. I would say a library rather than bookshelf, but just having 10 books would be rather impressive, and calling 10 books a library is a bit of a stretch.

OH!!! Wow i'm an idiot. It only just now occurred to me that you have a great opportunity here. There's another mod out there that adds the ability to equip belts, shields, and cloaks. However the mod itself doesn't create any of those artifacts, it's just the UI in the inventory that they were hoping someone else would use in creating the actual artifacts. Up to you whether you want to make use of it or not, but thought i'd mention it.

I see you added Hel. Thanks for taking my suggestion.

Textures and models aside, I would still recommend Masamune. What you could do is simply combine what I said about Musashi's twin blades with what I suggested about Masamune. The end result would be a sword that helps the wielder win duels (via special moves) and rewards winning duels with increased chance to get abilities like Blademaster. You could even add custom abilities.

@Cairsiden I know about that mod, and I've been thinking of using it, though not right now. The books will indeed have different rarities, with some being common, and others not so much.

@Mastigos If the texture isn't a problem, I could probably add it, though I'm reluctant to editing files, and to add more moves would require to edit how duels work. Also I don't think that there's a way to fire an event after a duel (without editing the duels), but I could make it give you a chance to become a Blademaster the longer you hold it.

@Stylianos_V That sounds good. After all, you can always edit it in the future if more modder tools come out.

can i request:

Coconut
as a court pedestal artifact thought funding adventure to North Europe region with follow description:
"Found them? In Mercia? The coconut is tropical!"
"Are you suggesting coconuts migrate?"

European Swallow
as a court pedestal/furniture artifact thought funding adventure to any Europe region with follow description:
"In order to maintain air-speed velocity, a swallow needs to beat its wings 43 times every second, right?"
"Its common knowledge about swallow as kings or queens to know these day."

African Swallow
as a court pedestal/furniture artifact thought funding adventure to Africa region with follow description:
"An African swallow may carry coconut by the husk, but this specie are non-migratory."
"Its common knowledge about swallow as kings or queens to know these day."

Suggestion: Tizona
We have a model for it in game, tried to make it an actual sword myself, but couldnt seem to remove the weapon-rack from the model

Gungnir (Odins spear) and Mjølner (Thors hammer), should give norse opinion and raid speed. Gungnir probably intrigue too and Mjølner stewardship.

There the mod on ck2 called WOWGAGS MORE ARTIFACTS AND BLOODLINES
bunch of artifacts on there as possible ideas for additions to this mod
love to see more ancient historical ones and mythical weapons and armour etc added
im not saying out right rip their mod from steam but good inspiration for you to add to yours