Which Wych Elm?

The answer to the titular question is: the wych elm in the garden of Ivy house, the one keeping a sordid secret-- but it's going to take some preliminary reading to learn this, and some of it isn't going to be pretty: brain damage, brain cancer, and a lucky, privileged young man brought to his knees by events from his past-- events that had consequences that he was oblivious to then but are horribly apparent now . . . I'll say no more, aside from the fact that The Wych Elm is another masterful mystery from Irish-American author Tana French.