Christiane Wolf provides a good introduction to appreciative joy or mudita. She also discusses some of the obstacles that can arise when we try to practice it. You can find her article here: How to Multiply Joy in Your Life
The University of Nebraska has a website for their employees on fostering well-being that includes a very good article on mudita (appreciative joy). It includes a good description of the quality and how to practice it in meditation and daily life. The article includes many links to written material and guided meditations on appreciative joy. You can find their article on mudita here: Mudita: A Meditation for Joy, Abundance and Others