A Comprehensive Manual of Abhidhamma - The Abhidhammattha Sangaha of Acariya Anuruddha — by Bhikkhu Bodhi
This is the classical introduction to the study of Abhidhamma - the Buddhist philosophy of mind and mental processes.
Gateway to Knowledge - The Gate for Entering the Way of a Pandita — by Jamgon Mipham
A condensation of the Buddhist Canon and its accompanying commentaries. Consolidating the intent of Buddha Shakyamuni's teachings into a unified body of text books, it is the philosophical backbone of the living tradition of Tibetan Buddhism.