The Abhidharmakośabhāṣyam of Vasubandhu, Vols. 1-4 (Trs. Louis de La Vallee Poussin & Leo M. Pruden, 1991)

Vasubandhu’s complete 4-volume text, first translated from Sanskrit to French by Louis de La Vallèe Poussin, then from French to English by Leo M. Pruden.  A classic of Indian Buddhist philosophy that is still carefully studied in the 21st century due its profound analysis of the life of the mind.

From the Preface: 

The Sanskrit word "Abhidharma" means the systematic philosophy of Buddhism. From the time of the Buddha onward, the Buddha's disciples, and many later generations of his followers studied, analyzed, and re-classified the teachings of the Buddha, and in the process created a unique field of study which has come to be known as the Abhidharma. 

The development of Buddhist philosophy —the Abhidharma— has continued to be developed up to the present day, especially within the field of Tibetan Buddhism. The early part of this Abhidharma literature —dating from the death of the Buddha to approximately the 5th century A.D.— is today preserved in Chinese translations, translations carried out largely by Hsuan-tsang in the mid-7th century; and the bulk of the later Abhidharma literature —dating from the 5th to the 12th century— is largely preserved in Tibetan translation. 

Only a small but important portion of this literature has been preserved in its original Sanskrit: Vasubandhu's Abhidharmakośabhāṣyam, and its commentary, Yasomitra's Vydkhyd. The student of the Abhidharma is therefore obliged to develop a reading ability in Buddhist Chinese, Tibetan, and of course, Sanskrit and Pali.