The group you're thinking of is the Hashashin (from which the word 'Assassin' was originally derived) and it wasn't Genghis Khan but Mongke Khan that they allegedly tried to assassinate. I'd probably try The Assasins by Bernard Lewis. There is a section on them in The History of Terrorism, edited by Gerard Chaliand, which I found a good read but is by no means specifically focused on the Hashashin. The Assassin Legends by Farhad Daftary also seems worth a look.