It rank as one of the most extraordinary correspondences of all time. But having fallen in love with a well-connected socialite 35 years his junior in the spring of 1912, Liberal prime minister HH Asquith, then 59, wrote obsessively to 24-year-old Venetia Stanley over the next three years. Up to three times a day, sometimes during Commons debates, even in Cabinet meetings, he penned 560 letters in total.

Their beguiling mix of passionate, sometimes mawkish,…