Half half. There's a bunch of 'what if?' chapters that aren't technically canon but are still worthwhile for character development and the like, but there are also backstory type chapters that are. And then there's the final chapter which is both definitely canon and incredibly good.Didn't know BoS wasn't considered canon. Makes it less of a big deal to me.
Really hope they do localise Book of Shadows, it's not quite as good as the original but it's still a great game overall.