I third Fifth Business. The description didn't pull me in right away, but issybird saying it's her favourite Canadian novel has me intrigued.
With my last nomination I had a hard time deciding between three books including The Stone Angel by Margaret Laurence (Canadian) and The Colony of Unrequited Dreams by Wayne Johnston (Canadian), but I finally chose (the one from the U. S.):
I nominate A Separate Peace by John Knowles.
Set at a boys’ boarding school in New England during the early years of World War II, A Separate Peace is a harrowing and luminous parable of the dark side of adolescence. Gene is a lonely, introverted intellectual. Phineas is a handsome, taunting, daredevil athlete. What happens between the two friends one summer, like the war itself, banishes the innocence of these boys and their world.
A bestseller for more than thirty years, A Separate Peace is John Knowles’s crowning achievement and an undisputed American classic.
"A masterpiece." - National Review
"A model of restraint, deeply felt and beautifully written." - The Observer