ACT requires much less "test prep" than the SAT. The ACT is not a "tricky" test, it just tests actual knowledge. There are no penalties for wrong answers.
I think the best idea is just to download some of the information and test prep info from the ACT site. You honestly don't need to buy a book, although they are going to try and sell you one. Is your child one that does well normally on standardized testing? If so, just being familiar with how the test works is the biggest hurdle. If they are comfortable with testing, have good knowledge in all courses, and understand the ACT process, they should do fine.