I'm looking for any novels where MC get's isekai'd into a game or book world and they make use of secret things from the series they got isekai'd into
In the Land of Leadale For an EN web novel: Memoirs of Your Local Small-time Villainess - probably one of the best fits; the MC transmigrates as a baroness who is a small-time villainess in an RPG, she then proceeds to use a lot of her game knowledge to find artifacts and get ahead. Fated To Fall: A Transmigrator LitRPG Tale Tori Transmigrated The Saintess and the Villainess
Isn’t “The Novel’s extra” this? I didn’t read it, but afaik, it’s an author that got isekaied in his own novel.
Jackal among snakes has the same premise as "Memoirs of Your Local Small-time Villainess": the main character reincarnates as a small-time villain, then steamrolls opposition with his metagame knowledge.