Barron has a book called Barron’s ACT 36: Aiming for the Perfect Score. Basically, it goes past the basic information presented in the other study guides and teaches you the most challenging concepts that may appear on the test, suited for someone with a top score looking to bump it up a little bit more.
I won’t try to tell you that it’s an absolute necessity, but it’s definitely an option for you to consider (personally, I think it helped me because I ended up with a 35 non-superscored).