The ACT and SAT are very different. For example, the ACT Math is just like the math your kids do in math class, but the SAT math is composed of tricky problems very unlike traditional schoolwork. Another example is the essay. The essay on the ACT is very down to earth/practical. The essay will resemble an essay from government/social studies class more than an essay from literature or philosophy. The ACT essays always pose questions that relate to high school kids in some ways, such as "Do you think school uniforms should be mandatory?", "Do you think kids should have to wait until they are 18 in order to get a driver's license?", and "Should grades be given in every class?"
Perhaps the biggest difference between the SAT and ACT, is that the ACT has a Science section. However, one need know little science in order to do well on it. The Science section tests the ability to read graphs, charts, tables, and diagrams, and the ability to see relationships between numbers. This section also tests reading comprehension, reading speed, and organizational skill, as well as the amount of practice doing ACT Science questions, one has had.
The book I recommend for ACT prep is "The Real ACT Prep Guide," a big red book that is the official guide to the ACT (see above). You'll want the 3rd edition which should have 5 practice tests.
The cheapest place to find the Real ACT Prep Guide is Amazon, NOT the official website. Compare my Amazon store prices (link above) to the Official ACT website prices.