Assassin book recommendations?

I'm looking for books that feature an assassin as the main character. I'm looking for more of an adult fantasy but I'm open to YA suggestions. I'd like to read something that focuses on the assassin, and the complex world they're apart of. Bonus points if they're apart of an organization of assassins. I'd prefer no romance, but if it's done well, I can work with it.

I just finished the Poppy War, which I enjoyed immensely, and I've also been reading some Batman comics focusing on Damian Wayne, which is why I've been in this very specific mood for an assassin fantasy.