GESHÉ LHUNDUP SOPA (1923–2014) introduced Dr. Apple to stages of the path (lam rim) theory and practice at Deer Park Buddhist Center in Oregon, Wisconsin.
He has published over fifty peer-reviewed articles in international academic journals. His books include Stairway to Nirvāṇa (State University of New York Press, 2008), A Stairway taken by the Lucid: Tsong kha pa’s Study of Noble Beings (Aditya Prakashan, 2013), Jewels of the Middle Way, The Madhyamaka Legacy of Atiśa and His Early Tibetan Followers (Wisdom Publications, 2018), Atiśa Dīpaṃkara Illuminator of the Awakened Mind (Shambala, 2019), and An Old Tibetan Dunhuang Manuscript of the Avaivartikacakrasūtra (Revue d’Etudes Tibétaines, 2021), and Atisha's Stages of the Path to Awakening (Wisdom Publications, 2025).