Here’s the deal: I only bought one prep book.
I was projected at a 28 from my lousy PSAT. I earned a 30. This was incredibly disappointing to me at the time, so I bought the Princeton Review’s “Cracking the ACT 2015” book. I used it religiously (like 1-1.5hrs/day) for the week leading up to the June 13th ACT.
That book bumped me from a 30 to a 34.
Barron’s may well be a better guide, I do not know, but I’d vouch for mine.