What is Reddit's opinion of ACT Prep Black Book: The Most Effective ACT Strategies Ever Published?

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It's listed in the sidebar. Here's a link to it on Amazon: https://www.amazon.com/ACT-Prep-Black-Book-Strategies/dp/0692027912

Additionally, here's a page with links to old official ACT tests (on a side note, prepscholar's blog tends to be a pretty good source in general, too. Also these are accessible for free): https://blog.prepscholar.com/complete-official-act-practice-tests-free-links

And the official practice test you can get for free from ACT: https://www.act.org/content/act/en/products-and-services/the-act/test-preparation/free-act-test-prep.html

Black Book (https://www.amazon.com/gp/product/0692027912/) which should be paired with the 2015 ACT official prep book.(https://www.amazon.com/Real-ACT-3rd-Prep-Guide/dp/076893432X/).

ACT Prep Black Book, Ultimate Guide to the Math ACT and Top 50 ACT Math Skills for a Top Score are used by lots of students and tutors. I won't recommend you reading the whole book though. Smart prep is all about finding your weak knowledge points first through practice and work on those specific skills through targeted and focused practice or reading related materials to fill the content gaps.