"Cannot we delude the eyes/ Of a few poor household spies?” Volpone sings to Celia in Ben Jonson’s play. On the evidence of these two books, the answer is “No.” Consider the fate of John Somerville, a Warwickshire Catholic who decided, in October 1583, to assassinate Queen Elizabeth. He was foolish enough to say as much before witnesses, describing the Queen as “a serpent and a viper.” Within days he found himself being interrogated by no less a person than Sir Francis Walsingham, the head of Elizabeth’s security service. He was found hanged in prison before he could be brought to trial for treason. This may have been a prudent forestalling on his part, to avoid the drawing and quartering that followed a traitor’s hanging, or he may simply have been disposed of without fuss.

This anecdote from Stephen Alford’s The Watchers is a small but...