I plan to seek out another author to work on a school counseling book. What should I do to protect myself from another author taking the idea before we complete the project and publishing it as their sole work?
It sounds like you're starting out on the wrong foot. To produce a successful book, it's important that you foster a spirit of collaboration and contribution with your co-author. One way to do this is to have an agreement up front. The agreement should state that both of your names will appear alphabetically on the cover, it should include a preliminary outline of the book, and should identify who will be the primary author of each chapter or segment.
It's likely that the two of you have different writing styles, so you should also discuss how you're going to achieve some consistency in the book. You can talk your way through this issue, select a style-guide, agree to peer edit each other's work. Again, the key thing is to work things out constructively and diplomatically ahead of time.
Although the book might not make much money, it could help both of your careers and create new job opportunities. If your collaboration doesn't work out, with the two of you parting ways and publishing separate books, you still get the same benefits from publishing an excellent book on your own. The existence of a book by another author won't affect your outcome. No matter what, you have a lot to gain, and little to lose.