In recent years the Taylor has had a leaning for big historical titles -- Andrew Preston on American religion, Tim Cook on the First World War, Richard Gwyn on Macdonald, Thomas King on aboriginal history -- or for memoir (Ian Brown on his disabled son, Andrew Westoll on living with chimpanzees), and it has traditionally had a weakness for any nonfiction written by a prominent novelist (Carol Shields, Wayne Johnston, Rudy Wiebe, etc.).
There is a bit of each in this list: Johnson and Taylor have written memoirs, Vassanji and Winter are novelists. I wouldn't have thought of O'Keefe's Dieppe book as the leading historical nonfiction last year (or indeed that there is an untold story of Dieppe!), but there's another historian's book on the list.
According to The Guardian, Barbara Taylor is a British historian of feminist and radical movements (she was born in Canada but has lived and worked in Britain). She has also lived with mental illnesses much of her adult life, and the memoir apparently blends personal experience with a history of changing treatments for mental disabilities.
Winner early in March.