[Lindsay Porter, author and cultural historian, has published widely on conspiracy theories and secret societies.]

Your first choice is Political Murder: From Tyrannicide to Terrorism.

This is the most historically far-reaching of all the books I’ve chosen, and raises interesting questions about the causes of assassination and the different attitudes surrounding it during different time periods and in different cultures. Although the book is organised chronologically, beginning with Classical Antiquity, and looking at notions of vengeance and judgment in the Bible, it continuously raises comparisons across time and place, whilst avoiding what the author describes as ‘laboured’ historical analogies. The result is always challenging and thought-provoking.

Ford looks at three different concepts of politically motivated killing in this book – with the broadest being political murder (from the specifically targeted to random killings), assassination and then, specifically, tyrannicide. He explores the origins and definition of the word ‘assassination’ and how that has evolved. The word is thought to have its origins in the Ismaili Muslim sect, the Hashishin, operating in the Middle East from 1090 to 1272, a sect which operated with a great deal of secrecy, including murder of political opponents. The Crusaders then picked up on it and the Hashashin gained a reputation for ruthless, covert killing. Marco Polo elaborated on the myth, describing a potent and irresistible blend of drugs and sex, in which recruits were drugged and promised an afterlife full of young maidens in exchange for unquestioning fealty. The notion of the assassin as working covertly, often as a result of a plot or conspiracy, had pretty much taken root by the 14th century.

Equally important with the evolution of the idea of assassination are the debates about the justification for politically motivated killing, and any study of assassination will look at the different philosophical debates trying to justify tyrannicide. A significant treatise debating the rights and wrongs of tyrannicide, Policraticus, was written by John of Salisbury, Thomas Becket’s clerk. One would expect a theologian to argue against any kind of murder, but John of Salisbury was responding to a debate that had been going on since antiquity, arguing the rights and wrongs of murdering a tyrant. A tyrant was defined as such either by the manner or his rule, or the means by which he had gained his power (tyrannus in regimine or tyrannus in titula). For John of Salisbury and many of his peers, God was the highest authority: if the king was against the law of God, his assassination was justifiable....