Sharon Salzberg is a gem! In my experience, lovingkindness meditation, or metta, is an essential life practice for peacebuilders. Loving self, friends, acquaintances, enemies - all are difficult. Thankfully, Salzberg provides very clear explanations for how one can move toward a life marked by greater love and compassion.
For Christians and other faith practitioners, metta offers a practical application of the Golden Rule and Jesus' radical teachings on loving enemies. Instead of jumping straight to loving enemies, Sharon insists that we start with loving ourselves. Otherwise, true love of enemies is impossible.
If the book resonates with you, you might also look into Salberg's audio guided lovingkindness meditations. I find Sharon's expert guidance through these meditations to be helpful for my own meditation practice.