The Māndukya Upanishad is one that is considered to be sufficient in providing Self realization/liberation if the seeker is ready. The most profound of the upanishads, it consists of only 12 verses, and yet convey the truth directly in a most powerful manner.
नान्तःप्रज्ञं न बहिष्प्रज्ञं नोभयतःप्रज्ञं न प्रज्ञानघनं न प्रज्ञं नाप्रज्ञम् ।
अदृष्टमव्यवहार्यमग्राह्यमलक्षणं अचिन्त्यमव्यपदेश्यमेकात्मप्रत्ययसारं प्रपञ्चोपशमं शान्तं शिवमद्वैतं चतुर्थं मन्यन्ते स आत्मा स विज्ञेयः ॥ 7 ॥naantahpragyam na bahishhpragyam nobhayatahpragyam na pragyaanaghanam na pragyam naapragyam,
adrishhtamavyavahaaryamagraahyamalakshanam achintyamavyapadeshyamekaatmapratyayasaaram prapanchopashamam shaantam shivamadvaitam chaturtham manyante sa aatamaa sa vigyeyah
Neither inward knowable, nor outward; not knowable from both ways. Neither unknown, nor knowable, nor unknowable. Invisible, non-interact-able, incomprehensible, not having any characteristics, inconceivable, non-mentionable, the substance of the conception of the single self, end point (of all), peaceful, auspicious, non-dual, deemed as fourth, he is self (Atma), he is to be known.