The Upadesasahsri is the best introduction to Sankara's philosophy and to Vedanta in general. The Upadesahasri consists of two parts, Metrical and Prose.
In the Metrical Part, Sankara discusses the basic philosophical problems of Advaita, at the same time refuting the teachings of other philosophical schools.
In the Prose Part he explains how to teach the means of final release from transmigratory existence for the benefit of students who are seeking after final release.
The Prose Part is a guide for the teachers, while the Metrical Part is a textbook for the students.