Reddit mentions of The Official ACT Prep Guide, 2016 - 2017

It's always best to go with the official study books. Here's the SAT and the ACT

Princeton Review is known for test-prep, but I've never used them, so I can't attest for quality... I just know that the official guides from the people who make the test are always your best bet for prep.

Agreed. By the way, the new ACT book comes out at the end of the month. It can be pre-ordered here.

Purchase and read through this book, thoroughly. Then do the practice tests in it:
https://www.amazon.com/Official-ACT-Prep-Guide-2016/dp/1119225418/ref=sr_1_1?ie=UTF8&qid=1475214343&sr=8-1&keywords=red+book+act


Approach questions with confidence after you read the book.

E: Learn the English grammar rules: http://blog.prepscholar.com/act-grammar-rules

M: Brush up on math with Khan Academy SAT Math Practice.

R: Practice tests!

S: Practice tests!

• When taking practice tests, always take them in a realistic environment (preferably in the AM).
  
  
• USE A TIMER. Practice setting the timer 5 minutes early. This will help you get used to 5-minute warnings. Try to finish AT the 5-minute warning so you have time to go back and check.
  
  You'll do great!
  

Also, if out of sources, order the the new red book.

This will have 3 additional recently administered exams. So make the best use of them!