Grim Expectations: The George Dower Trilogy Vol 3 New edition

Some time after the events of Fiendish Schemes, George Dower finds himself a widower, of sorts. On her deathbed, Miss McThane entrusts Dower with a small, ticking clockwork box. The box is mysteriously linked to her. When she breathes her last, the box stops ticking and Dower is able to open it, to find hundreds of letters - written in an unknown hand, signed only with the initial S. They're not love letters, but refer instead to the letter-writer's ongoing search for some other person. The last is a simple note, reading "Found him"...