The Viveka Chudamani, literally "The Crest-Jewel of Wisdom" is a famous work by Adi Shankara that expounds advaita vedanta philosophy. Having written commentaries to the Upanishads, Bhagavad Gita and Brahma Sutras Adi Shankara composed many sub-texts in simple Sanskrit, called Prakarana Granthas, with the objective of reaching the message of the Vedas and Upanishads to laypersons. The Viveka Chudamani, as the name implies, is the crown jewel of such texts.
More Info: Translation of Vivekachudamani