The very last letter, the «s» in ecosystems, gives it away. (clivehamilton.com)
The project is officially called «02.02.1861, [last letter of Saint Théophane Vénard to his father before he was decapitated](2009 ---RRB-.» (sfaq.us)
This is more familiar territory for Moyes; Last Letter From Your Lover, 2011, also features a double love story, set years apart and linked by a letter. (earlyword.com)