I just took my test last Thursday and scored a 90%.
All I used was the ATI test prep book VI edition and I bought the two practice test A and B on the website. Also there is a free app on the playstore for ATI Exam Prep VI, it really helped a lot when it came down to harder questions that the science portion may ask because the book doesn't go into as much depth.
I read through the book, went through the app, and took the apps practice tests and then did the practice tests on the website. I studied in a 3-4 week time frame.
You can probably get the book at a library or from someone who has already taken the test (like me), maybe even look on Amazon
for used versions. But the two tests on the site cost I think 80-$100.