I did both ATI and NCLEX practice questions since ATI produces the NCLEX as well. Read all the rationales, even on questions you get right and you should do fine. For me reading the book wasn't as helpful in retaining the information.
My school did ati for all ending exit exams as well as THE final exit exam. I have repeatedly found the best way to study is doing the practice test and paying special attention to the rationales. Seriously taking each practice test 3 and 4 times is the way to go. Then I like to scan through the ati book for topics we may have not covered in classes....like the eye. There's always stuff on ati about eyes. I think the OB was the easiest and most straight forward test of all.
Do ALL the practice exams, and do them more than once. Then read the books.
Also, do the Nurse Logic modules. I blew them off the first 3 or 4 semesters and really wish I hadn't. They really do help, and not just with ATI. You'll come to see the way ATI priortizes things, and to practice applying the nursing process to your questions. If I don't know something, or can't decide which answer is the MOST correct, I'll fall back on the nurse logic and it'll usually help. For example, ATI prioritizes ABCs first, then general safety. That is what the NCLEX is all about, too - whether you will be a safe nurse.