I want to apply and get accepted in the top colleges for undergrad degree:)
There is no single book or course that I can recommend to help you because let's face it, we all have different learning styles and inherent biases that are sometimes debilitating. For the ACT, I found that learning from books that used actual previous test questions was more helpful to me since succeeding on the ACT is not about content but how to take the test itself. If you are taking practice tests from past exams, then what can be better than that? Nothing. I found books that are not Official ACT workbooks from ACT.org augment the learning but they do not replace the need to drill into the actual examples of test problems and answers.
Some will disagree and say they got a 35 or 36 with Princeton Review or Barrons or the like. For me, I prefer the original source data. You can purchase them directly from ACT.org or Amazon or book stores. I wouldn't recommend buying a used copy because you want the ability to write notes in your own book and use the perfectly blank answer sheets, not those that were used by previous testers.
There is no Khan Academy equivalent for the ACT as the SAT so if you require some online course that will not break the bank, I would highly recommend SuperTutorTv's best ACT course. First I did the 5-day free trial to see if I liked it. I did. This course helped me the most to get from say a 98% percentile score to a 99.7% percentile score. If failure is not an option to get into a top Ivy or Elite, then you need to pull out all the stops and get the tweaks to help master how to attack the test.
I agree with @CameronBameron! Using questions from actual previous ACT tests will probably be one of the most helpful things you can do to prepare! The ACT tests you on how well you can do the ACT, not how smart you are. There are free resources online that allow you to practice with actual ACT questions.
^I used this link to study for my tests (it comes from the official ACT website and allows you to choose if you want it timed or time and a half)
As for the Princeton Review prep book, I also primarily used that when I was studying. The questions and texts used in the Princeton Review are very close to the actual ACT. I think the Reading and English sections in the Princeton Review were very accurate. The Math section was also pretty accurate. The Science section is the only one that wasn't exactly the same as the Official ACT. The Science section in the Princeton Review is much harder than the science section in the Official ACT imo. This could actually help you - when I retook my ACT, I practiced with the Princeton Review and the Science section on the actual test date felt like a breeze since I was used to the much harder Princeton Review questions.
No matter what you practice with, any practice will do you good! If I could do it all over again, I would probably practice with the Offical ACT book but I got really high scores using the Princeton Review book anyways (33 composite)! Good luck with your tests!
To keep this community safe and supportive: