As a therapist, I recommend numerous books and psychological tools. Some are suitable only for specific issues and others, like The Four Agreements by Don Miguel Ruiz, are a useful framework for a diverse array of problematic thinking. Whether a person suffers from mild jealousy or crippling anxiety, relational conflict or loss of direction, The Four Agreements provide a template for accountability and lasting change.
The Four Agreements are written in an accessible manner and complement a range of spiritual orientations (or no orientation). Deceptively simple, they take a lifetime to master and create a backbone to the therapeutic process which I have utilized with dozens of clients. You can read the book quickly, join online forums, or embark on one of the in-depth trainings that take place worldwide.
Curious? Check out The Four Agreements here for an overview.