The Ismā‛īlīs are a sect of the Shī‛ī branch of Islam. They are regarded by many in the west today as epitomizing Islamic 'gnosis', and there is no denying the similarities between their beliefs and those of the more esoteric versions of Judaism and Christianity. They are considered to be terrible heretics, however, by many orthodox Muslims, both Sunni and Shī‛ī, and have been subject to awful persecution at different times in their history. They are most well-known as the sect of the medieval 'Assassins', and their political clout and numbers were both quite high in the Middle Ages. Today, however, they are a small community, or rather, a number of scattered communities, located mostly in Iraq, Iran, and India.
Until twenty or thirty years ago, there was very little study of the Ismā‛īlīs in the west. That has begun to change drastically, however, and anyone interested in learning more about them should be able to find material with very little trouble. The work of Farhad Daftary, an Ismā‛īlī himself, as well as a great scholar, is a good place to start. The files indexed below are by Edward E. Salisbury, and were originally published in The Journal of the American Oriental Society. The first two files (after the Introduction) are actually anti-Ismā‛īlī tracts by orthodox heresiologists. The last two files are expositions of Ismā‛īlī doctrine, apparently from divergent sources. These may be the only source material available in the public domain in English. Any notices of additional available material would be greatly appreciated.
JAOS, vol. ii, art. xii, pp. 257-324:
JAOS, vol. iii, art. ii, pp. 167-193: