Do the 'Historical Notes' in the Handmaid's Tale add to or detract from the novel?

The 'Historical Notes' featured at the end of the Handmaids tale explain several un-answered questions about the book. There has been much debate on the subject of the Historical notes and whether they add or detract from the novel. The Historical notes take away the mystery of the book. The last line of the final chapter reads 'And so I step up, into the darkness within; or else the light.' This is a really …

…the story. However, this isn't enough to counter the fact that the historical notes dismiss any sort of uncertainty as to the fate of Offred as it almost confirms that she did escape. I think the mystery that surrounds the end of the book is what makes it special. Unfortunately, the formal end to the book in the form of a speech ruins the atmosphere that was created by the narrative structure of the novel.