I think the sections in the ATI book are similar to the actual exam, with the exception of the science section. The ATI book doesn't go into the detail that you need for the science section so I would supplement your studying material. When I took the science sections in the ATI book, I scored around 90, then I checked out the McGraw-Hill TEAS Exam book (just 5 TEAS exams and brief descriptions of answers) from the library and scored 68% on the first two science sections. I started to get anxious, but I just reviewed my weaknesses like crazy, using Chemistry for Dummies, my biology & anatomy books, and khanacademy.org. On the last two exams I scored 95% and 80%, respectively, and ended up getting an 85% on the online science section. Honestly, it's a hit-or-miss unless you remember every detail of your chemistry, bio, A&P and microbiology classes. If you have the ATI manual memorized, I would say at best you would score an 85%. The exam goes into some details on some systems, photosynthesis/cellular respiration (what's oxidized &/or reduced, what's anabolic vs. catabolic), translation/transcription (where does each take place, the details of transription process within ribosomes, using condons/anti-codons to identify amino acid sequence), and chemistry.
The online exam is harder than the actual exam, but it gives you a good idea on how to pace yourself. What I didn't like about the online exam though is that it reviews the answer after each question with only one question per page. On the actual exam, there are multiple questions per page.
For the other sections, if you really understand what's in the ATI book, you will do well.