They are as follows: Almost all printed editions of ancient Vedas and Upanishads depend on the late manuscripts that are hardly older than 500 years, not on the still-extant and superior oral tradition.
This ensured an impeccable textual transmission superior to the classical texts of other cultures; it is, in fact, something like a tape-recording.... Not just the actual words, but even the long-lost musical (tonal) accent (as in old Greek or in Japanese) has been preserved up to the present.
these form the core of ancient texts, predating classical Hinduism; they span the 1st millennium BCE and reflect the emergence of Vedanta from Vedic religion.
Īṣa Bṛhadāraṇyaka Kaṭha Taittirīya Śvetāśvatara Praśna Muṇḍaka Māṇḍūkya Kena Chāndogya Maitrāyaṇi Kauśītāki Aitareya These are general Upanishads, and do not focus on any specific post-classical Hindu tradition.
[15] Subāla Mantrikā Nirālamba Paiṅgala Adhyātmā Muktikā Sarvasāra Śukarahasya Skanda Śārīraka Garbha Ekākṣara Akṣi Prāṇāgnihotra Sūrya Ātmā Vajrasūchi Maha Sāvitrī Ātmabodha Mudgala These are Upanishads that focus on renunciation-related themes and the life of a sannyasi (monk) Jābāla Paramahaṃsa Advayatāraka Bhikṣuka Turīyātīta Yājñavalkya Śāṭyāyaniya Brahma Tejobindu Avadhūta Kaṭharudra Nāradaparivrājaka Paramahaṃsa parivrājaka Parabrahma Āruṇeya Maitreya Sannyāsa Kuṇḍika Nirvāṇa These are Upanishads that focus on goddess Devi-related themes Sarasvatīrahasya Sītā Annapūrṇa Devī Tripurātapini Bhāvana Tripura Saubhāgya Lakshmi Bahvṛca These are Upanishads that focus on god Vishnu-related themes Tārasāra Nārāyaṇa Kali-Saṇṭāraṇa Nṛsiṃhatāpanī Mahānārāyaṇa Rāmarahasya Rāmatāpaṇi Gopālatāpani Kṛṣṇa Hayagrīva Dattātreya Gāruḍa Vāsudeva Avyakta These are Upanishads that focus on god Shiva-related themes Kaivalya Kālāgnirudra Dakṣiṇāmūrti Rudrahṛdaya Pañcabrahma AtharvashirasAtharvaśikha Bṛhajjābāla Śarabha Bhasma Gaṇapati Rudrākṣa Jābāli Akṣamālika (Mālika) These are Upanishads that focus on Yoga-related themes Haṃsa Triśikhi Maṇḍalabrāhmaṇa Amṛtabindu Amṛtanāda Kṣurika Dhyānabindu Brahmavidyā Yogatattva Yogaśikhā Yogakuṇḍalinī Varāha Śāṇḍilya Pāśupata Mahāvākya Yogachūḍāmaṇi Darśana Nādabindu