Buddha's Lions - The Lives of the Eighty-Four Siddhas — by Abhayadatta
Life stories of eighty-four Tantric masters who achieved the highest results of the Vajrayana. Excellent introduction to the Vajrayana and a good resource for the study of Tibetan.
Masters of Mahamudra - Songs and Histories of the Eighty-Four Buddhist Siddhas — by Keith Dowman
These eighty-four masters - some historical, some archetypal - accomplished this practice in India where they lived between the eighth and twelfth centuries. Leading unconventional lives, the siddhas include some of the greatest Buddhist teachers - Tilopa, Naropa, and Marpa among them.