Hey, I have taken the test 3 times. The first time I did not study at all. I got a 78. Second time, I only studied for 3 hours. Got a 79. The third time, studied for about 3 days and got a 92%. I really do recommend the ATI book.
If you aren't fresh on most of the material and are barely getting any study in, I would suggest you be prepared to probably have to take it an additional time.
Get the ATI book for sure. For the Reading, Math, and Language/ Grammar
portion the book is pretty spot on. As for the science, it's pretty much a gamble. The book touches base on the topics, but not very specific. It's more broad concepts. Youtube, is good source to get into a little more depth of the concepts.
The math really is pretty basic, but just make sure you're able to solve the problems properly and quickly.
Go through the book and read it a couple of times. Just highlight and reread it as often as you can. It really helps and will help you remember the information.
If science isnt your best subject, study up on that most, but ALSO study up on other subjects a lot too. If you can score really high on the other portions and at least do okay in science you should be able to score pretty high. I got High 90s in everything else and got 80 something in science the third time around.