Gil Fronsdal’s book “The Issue at Hand” has a couple of chapters on metta. One describes the qualities of metta, and the second is a brief introduction to metta practice in meditation. You can find those chapters here: The Issue at Hand Chapter 21:Metta And here: The Issue at Hand Chapter 22: Loving-Kindness Meditation
Mark Nunberg has a good description of the practice of lovingkindness and its potential impact on our lives on this page: Practices of Lovingkindness The page also includes a number of audio and written resources on lovingkindness, including links to guided meditations from several sources.