Mods? This game can be moded? How mod friendly is it. like bethesda games?
Larian released a quite powerful tool to mod the main campaign and create standalone modules. And many game files can be edited. But for a couple of reasons (Editor to be updated, delayed Editor release, no documentation, no actual support, some basic things hardcoded etc.) Divinity modding didn't really grow, despite its big potential. D:OS 2 will have dedicated mod support and better tools, so we can hope for more.
Back to topic: I have a recipe book transfer to separate this type of book from other books and a transfer function for quest items. As far as the devs declared all quest relevant books as story items you can separate story relevant and other books quite easily. But that's just an offer. Your main problem seems to be identification of story books which is quite impossible. But most books should be sellable after you read them (if not all books).
And as Azothoth suggested: improve your inventory management, use containers, barrels, baskets, backpacks, chests to store items at a location of your choice (inside or outside your inventory).
