I'm looking for a supplementary prep book that I can use after the official ACT red book, but I'm not sure which one I should get. What's the best option?
I'd recommend the Barron's ACT prep book as a good secondary resource. It's a thorough book that has lots of strategies, high-quality content, and practice questions. The one slight issue is that the questions tend to be more difficult than the questions on the real ACT, but since you've seen the official practice questions in the Red Book, you'll probably be aware of this and won't end up misjudging your score potential. This article has a list of more books that you might find helpful: http://blog.prepscholar.com/the-10-best-act-books-recommended-for-act-prep.