"Double Agent" Books

Hi everyone I recently read The Will of the Many and I loved the fact that he goes "undercover" among his enemies pretending to make friends with them (at least that was the initial goal)

I was just looking for books where really the main character does a double act where he pretends to be nice and make friends with everyone but he never changes his mind, he never or almost never says "ah but he is different".

Something very Machiavellian.