Page 232. " But when the Ides of March were come (of which I do remember something dim from school, and something clear from my favourite writer) "
William Shakespeare
Public DomainWilliam Shakespeare - Credit: National Portrait Gallery

The Ides of March is 15th March in the Roman calendar.  It was a festive day dedicated to the god Mars.  It was on this day that Julius Caesar was murdered in 44B.C. On his way to the Theatre of Pompey, Caesar encountered a seer who had foretold that harm would come to him not later than the Ides of March. Caesar joked, "Well, the Ides of March have come", to which the seer replied "Ay, they have come, but they are not gone."  Caesar’s downfall is dramatized in William Shakespeare’s play Julius Caesar, in which Caesar is warned to "beware the Ides of March.” 

John Ridd mentions several times in the novel that Shakespeare is his favourite writer