Just finished James Waterson's The Ismaili Assassins. This is the story of a religious sect based in the Middle East that waged war primarily through assassination.
I picked the book up some months ago having heard of them, but without any real understanding of their role during the medieval era. The author explains the religious and social context before setting out their impact on the empires of the period. They were largely successful in the early period and in their conflict with the disunited Seljuk's. However, as the initial fervour declined they became less effective before finally picking an unwinnable fight with the Mongols. Interestingly, I didn't know that the sect's modern day successors include the Aga Khan.
This is not a light read by any means, but worth the effort to dispel some myths and learn more about them.