Susiddhikara Tantra 

An early kriyā-tantra of great importance in approximately 38 chapters, describing in great detail the preparations and methods to be used by a mantra-practitioner to guarantee the successful attainment of accomplishments (siddhi). Knowledge of the information given in this text was widely assumed in tantric Buddhist practice in Tibet and among the esoteric schools (see Esoteric Buddhism) of China and Japan. Though no Sanskrit text survives, translations are currently available in both Chinese and Tibetan.

A Dictionary of Buddhism, Oxford University Press, 2003, 2004 (which is available in electronic version from answer.com)