What kind of text they are
The Upaniṣads are not systematic philosophy. They are dialogues, parables, dialogues-within-parables, terse koan-like exchanges, and occasional longer expositions. The student-teacher form recurs: a young man approaches a sage with a question (who am I?, what is brahman?, what happens to the self at death?), and the answer is given gradually, through analogy and progressive reframing. The Chāndogya, Bṛhadāraṇyaka, Kaṭha, Īśa, Muṇḍaka, Māṇḍūkya and Praśna are the most-read in English.
The great statements
The four mahāvākyas — drawn from four different Upaniṣads, one per Veda — became the doctrinal compression of advaita Vedānta: prajñānaṁ brahma (consciousness is brahman, Aitareya), aham brahmāsmi (I am brahman, Bṛhadāraṇyaka), tat tvam asi (that thou art, Chāndogya), ayam ātmā brahma (this self is brahman, Māṇḍūkya). The point of the four is not memorisation; the point is that they are pointing at one identity from four angles.
Translations
Eknath Easwaran's The Upanishads (Nilgiri Press) is the most accessible serious modern translation in English. Patrick Olivelle's Oxford World's Classics edition is the rigorous scholarly version. Juan Mascaró's Penguin Classics translation reads as poetry but is more interpretive. For the contemporary non-dual seeker, Easwaran is the right starting point; Olivelle is what you graduate to when you want to know what the text actually says.
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