For a book of simple and excellent practice, a book to calm the heart and focus the mind through Metta practice (don't be put off by the word) I am recommending Boundless Heart: the Buddha's Path of Kindness, Compassion, Joy and Eqanimity by
Christina Feldman. Yes, a Buddhist practice but the simplest and most practical application - and the most needed, I might add - in the turbulence of these times. Not only the very short practices, but the introductions and the reflections are very very helpful
in the turmoil of today. I know of no better recommendation - the turbulence of these times in the world call for it.