I frequently make study materials recommendations to my students. I don't recommend the same thing to each kid, and I might recommend different materials for students working with a tutor than I would to students working on their own. However, there are some general guidelines I would apply to everyone:
First, you want the "official" study guide.
For the ACT: The REAL ACT Prep Guide
For the SAT: The Official SAT Study Guide
These are the ones written by the same people who write the test. They are generally reasonably priced given how thick they are, but you can often get access to a "free" copy from a friend or the library. Try to find one in which the answers haven't all been circled in the book. If you do have to buy one, make sure to answer the questions on another sheet of paper, and then pass your copy on to a friend or the library.
For some people the official guide will be enough. However, many students take a practice test and discover that they need extra help with some particular thing. The official guides do include some review material, but it is not necessarily written in a user-friendly way. What you are looking for here is a study guide that includes targeted practice. In other words, once they teach you how to do something, there should be an opportunity to practice it right then. You shouldn't have to pick through practice tests trying to find the types of items you want to practice.
You should be able to leaf quickly through the book to tell if the guide will have targeted practice. If all of the practice questions are at the end, put it back on the shelf. You don't need this person's version of a full practice test. That's what you got the official guide for. You will probably want to actually buy a copy of some of these books because scribbling in them is part of the process. Conversely, a free copy could be useless if the scribbling has already been done by someone else.
A sample of books with targeted practice:
The Top 50 Skills for a Top Score series (includes books for both the SAT and the ACT) by Brian Leaf - these are perfect if you need help for a test that is less than a month away, but less good if you need a really high score
*The Ultimate Guide to SAT Grammar by Erica Meltzer
*The Critical Reader by Erica Meltzer
McGraw Hill's Conquering SAT Writing
Barron's Grammar Workbook for the SAT, ACT and More
*PWN the SAT Math Guide by Mike McClenathan
Kaplan SAT Math Workbook
The Gruber's books (includes books for both the SAT and the ACT)
An * marks the books that will probably need to be ordered. The others are often - although not always - on the shelves at your local big box book store.