Wait I think I almost got it but can you rephrase it a little?
Do you mean an NPC who owns a potion shop and finds a book about the world she lives in, and the book has information about people/secrets/treasures? And there’s a prince + a hero who got killed by villagers? ∘ ∘ ∘ ( °ヮ° )
Also, this might be completely wrong, but could it be Trapped in a Cursed Game as an NPC? I can’t really remember anything about a book (at least at the start), but the FMC does know stuff about the game like secrets/treasures and I’m pretty sure the prince is the ML… so potion shop NPC made me think of that immediately (ᵕ—ᴗ—)

An npc who own a option shop? Findes a book about the Worldtrade she lives in, which has information about people and their secrets and treasure aswell. There’s this Prince? And this hero who was Billie by the villaigers??