The 55-year-old ’flu-ridden John Charles Wallop, 3rd Earl of Portsmouth, his feet in a basin of warm water, shivered in the dock with fever but also with fear. Would the jury, assembled in 1823 in London’s jam-packed Freemason’s Hall at the end of an unpredecentally sensational two-week trial, find him eccentric, delusional, simple-minded or, instead, stark raving mad?

Elizabeth Foyster, a historian and senior lecturer at Clare College, Cambridge, was alerted to this enthralling, almost unbelievable true story with the caveat that the vastness of the material in Lambeth Palace archives concerning a scarcely remembered trial of a hugely wealthy, relatively...