In vanilla, if you use overhigh level on anvil, it can only apply the highest level. But if "Ancient Tomes" is true, every level can be applied.

1. Run command /give @s minecraft:enchanted_book{StoredEnchantments:[{id:"minecraft:unbreaking",lvl:5s}]}
2. Apply it to a diamond pickaxe on an anvil, you can see the unbreaking V enchantment is keeped, costing 10 level
3. Set "Ancient Tomes" to false in config and restart the game. Apply that book again, unbreaking V enchantment will get lost and leave unbreaking III

Minecraft 1.16.5 Forge 36.2.35 Quark r2.4-322

I'm not sure what the issue is you're describing here? If you are giving yourself a book with commands it will always bypass any restrictions as nbt is not validated. i.e. You could reproduce the last step without even having Quark installed

I'm not sure what the issue is you're describing here? If you are giving yourself a book with commands it will always bypass any restrictions as nbt is not validated. i.e. You could reproduce the last step without even having Quark installed

You may misunderstand. In vanilla, enchanted book are always restricted. You CANNOT use enchanted book with overhigh level, no matter it's from command or another way, (diamond sword + unbreaking IV = diamond sword with unbreaking III). But if you install Quark, you CAN use overhigh level enchanted book unexpectedly.(diamond sword + unbreaking IV = diamond sword with unbreaking IV).

I mean that's one of the ancient tomes uses, to make enchanted books with a higher level than normal, you need to be able to combine an item with a over-enchanted book to get an over-enchanted item

I mean that's one of the ancient tomes uses, to make enchanted books with a higher level than normal, you need to be able to combine an item with a over-enchanted book to get an over-enchanted item

Ancient tomes can only make enchanted items have the higtest level+1 (diamond sword with unbreaking 3 + ancient tome with unbreaking 3 = diamond sword with unbreaking 4). That is in normal circumstances.
But this bug means you don't need ancient tome to get enchanted items with overhigh levels, what's more, its level are not restricted. In vanilla, an overhigh level enchanted book can only apply the highest level to the items, but if quark is installed, an enchanted book with unbreaking 255 can be applied to an item an the anvil, that's the bug.

How do you expect to get books higher then the max level in vanilla? quark doesnt magically let those books spawn, it only raises the max limit so tomes work

How do you expect to get books higher then the max level in vanilla? quark doesnt magically let those books spawn, it only raises the max limit so tomes work

Tomes do works as expected. But quark removed the restriction of enchanted book unexpectly. Any level of enchanted book can be used on anvil.
If you follow the steps at the top once, you'll understand.