For a good book on the Assassins, who Chingis Khan's grandson Hulegu pretty much wiped out in the 1250s see The Assassin Legends: Myths of the Isma'ilis by Farhad Daftary. For an on-line history see "The Assassins of Alamut". For what motivated the Isma'ilis see their Secret Doctrines. Although their castles were destroyed by the Mongols the Isma'ilis have survived to the present day, most notably in India but with enclaves all over the world. Their current leader is the Aga Khan, the 49th Imam and a noted philanthropist.