Pali text:

Dines Andersen and Helmer Smith. Sutta-Nipàta. Pali Text Society (1965). Uragavagga (p. 25-6).

Translations:

Aronson, Harvey B. Love and Sympathy in Theravàda Buddhism. Motilal (1980).

Basham, A.L., in W.T. de Bary. The Buddhist Tradition. Vintage Books (1972).

Gunaratana, H. Bhàvana Vandana: Book of Devotion. Bhàvana Society (1990).

Khantipalo, Bhikkhu. Calm and Insight. Curzon (1981).

Nanamoli, Bhikkhu. [1] The Minor Readings (Khuddakapàñha). Pali Text Society (1978).

Nanamoli, Bhikkhu. [2] The Life of the Buddha. Buddhist Publication Society (1972).

Narada, Maha Thera. The Buddha and His Teachings. Singapore BMC (1987).

Norman, K.R. The Group of Discourses. Pali Text Society (1984).

Rahula, Walpola. What The Buddha Taught. Gordon Fraser Gallery (1959).

Saddhatissa, H. The Sutta-Nipàta. Curzon Press (1985).

Thanissaro, Bhikkhu. A Chanting Guide. Dhammayut Order USA (1994).

Woodward, F.L. Some Sayings of the Buddha. Oxford UP (1973).

Recommended reading:

Ehara, N.R.M., Soma Thera & Kheminda Thera. The Path of Freedom (Vimuttimagga) by the
Arahant Upatissa. Buddhist Publilcation Society (1977). Chapter VIII, Section V.

Nanamoli, Bhikkhu. The Path of Purification (Visuddhimagga) by Bhadantàcariya Buddhaghosa.
Buddhist Publication Society (1975). Chapter IX: The Divine Abidings.

Nanamoli, Bhikkhu. The Path of Discrimination (Pañisambhidàmagga). Pali Text Society (1982).
Chapter XIV: Treatise on Lovingkindness.

Nanamoli, Bhikkhu. The Illustrator of Ultimate Meaning (Paramatthajotikà) by Bhadantàcariya
Buddhaghosa. Chapter IX: The Lovingkindess Discourse.

Thinnila, Pañhamakyaw Ashin. The Book of Analysis (Vibhaïga). Pali Text Society (1988).
Chapter XIII: The Illimitables.

Salzberg, Sharon. Lovingkindness: The Revolutionary Art of Happiness. Shambhala (1996).