The ACT for Bad Test Takers is a must-have if you're prepping for the ACT. It's basically a battle plan designed to raise an average score (teens/low 20's) to the higher 20's. But really, anyone taking the ACT can benefit from it if they take the strategy laid out in the book seriously.
But yes, to avoid any confusion: this isn't a "prep" book in the sense of having "prep questions" in it. It's a strategy book. Which is actually why you should read it.
Reading BTT is like having a conversation - a realistic, straight-to-the-point, "here's what you need to know to do well on the ACT" conversation with a team of professional tutors. They tell you how to approach each section, tell you what tradeoffs you should and shouldn't make (e.g. quality vs. quantity), and tell you how to prepare yourself on the day of the exam. If you're good at standardized testing, some of it may be familiar ground. If you're not good at standardized testing, then this book is honestly like a survival guide when you're stuck in the wilderness. But either way, you probably don't know the strategy that's laid out in this book, because no other book approaches the ACT (intelligently) in this way. Thus, you should read it.
Price-wise, it's a lot less expensive than hours of tutoring from agencies who may or may not know how to do well on the ACT. You can get through this book in 3-4 hours, so if you see it as paying $5 an hour for an empirically proven strategy, rather than paying hundreds for an agency you've never heard of, who happens to have hung flyers in your school hallway, to talk at you in a crowded gymnasium -- this is a really good deal. Side note: because the book is written by tutors, it also talks about how to be a smart consumer when hiring tutors.
It's also a book you can freaking read. Which may sound self-evident, but as someone who has spent hours beating their head against a desk trying to get through thicker "prep books" that don't actually say anything about what you are supposedly "prepping" for, I appreciate a book that's meant for high school students to pick up and get through without any trouble.
Last thing: the hidden gem in BTT is the FAQ section, which has questions like: What if I take the test tomorrow? How many times should I take the ACT? How do I build up my stamina for the ACT? Should I take the ACT or the SAT? Can I skip more than the recommended number of questions? (This question relates to the strategy of the book.)
So, give this book a chance. I think you'll be pleasantly surprised.