I think I might have been the one who mentioned the test book. DS will be taking his first ACT in Saturday. We got the book to make sure that it would be worth giving up a Saturday morning to take the test (Ania--I hear you. When they get to this age, there seems to be some activity every weekend and it's hard to decide how to prioritize). He has also never taken a timed test so I wanted him to take the practice to get a sense of how to budget his time.
He has taken 2 practice tests (one in Sept. when we were deciding and another in Nov. because the first one was so much fun!). The scores were close, but the second practice was a bit higher in reading and science, which is what I'd hoped for. But I can't tell you how well the practices will predict the real test.
