The foundation texts are the essential Buddhist texts on which almost all subsequent teachings are based.
From the Pali tradition: suttas and commentaries including the Dhammapada, the Samaññaphala-Sutta, the Ānāpānasati Sutta, and the Mahāsatipaṭṭhāna Sutta; key teachings like the Four Noble Truths; stories from the life of the Buddha; and essential guides to samatha and vipassana practice based on the canonical texts.
From East Asia: sutras like the Heart Sutra and commentaries, the Diamond Sutra, the Flower Ornament Sutra, the Samdhinirmocana Sutra, the Hui-neng (Platform) Sutra, the Sutra of Complete Enlightenment; important expositions including by Hakuin and Dogen’s Treasury of the True Dharma Eye; koan collections such as the Blue Cliff Record, the Book of Serenity, and that of Master Yunmen.
From the Sanskrit Indian tradition: the Five Maitreya Texts of Asanga; other key texts studied today including those by Nagarjuna, Aryadeva, Chandrakirti, Shantideva, Shantarakshita, Vasubangdhu, Strimati, and more.
From the Tibetan tradition: comprehensive overviews such as the Treasury of Knowledge and The Complete Nyingma Tradition; important Lamrims or Stages of the Path by Tsongkhapa, Gampopa, and Jigme Lingpa; Ngondro commentaries by Patrul Rinpoche and Dudjom Rinpoche; Essential overviews by Longchenpa, Ronzompa, Mipham Rinpoche, and more; and key Tantric expositions by Rangjung Dorje, Khenpo Ngawang Palzang, Sera Khandro, Dodrupchen Jigme Tenpa'i Nyima, and Lama Dampa Sonam Gyaltsen; and key tantras including the Guhyagarbha and Kunjed Gyalpo.