I’m not sure how you’d define “great book.” Two of my highest-rated readings lately have been John McCain’s The Restless Wave: Good Times, Just Causes, Great Fights, and Other Appreciations and Bob Woodward’s Fear: Trump in the White House. They’re both very strong books and very much worth reading, but I wouldn’t call them “great.”
(You asked “when.” Those were July and September, respectively.)
If you mean classics, the most recent was probably Lord Jim, by Joseph Conrad, which wasn’t easy but did have the virtue of not being Heart of Darkness. (May.)
So far this year, though, my highest rating went to Leonardo da Vinci, by Walter Isaacson. It was one of those books that opened chambers of the mind, or so it seemed to me. (January.)
In general I read about twice as much fiction as nonfiction, but for me this has been a year that calls for realism over escapism, much good may it do me.