iPhoto book - clear placed photos

I had almost finished an iPhoto book but I was trying to delete the photos that I had already placed from the list of available photos on the right end pane and clicked "Clear placed photos". To my horror, all photos that I had placed in the actual book were deleted and now I have a completely empty book. Please, how do I go back to the previous version? Command Z doesn't do anything. I've tried to look for saved books but I can't find it. Please help me, I spent days doing this book. Thank you