One annoying aspect of iBooks is how much whitespace is wasted on the margins of the body text. Applying background colors (cyan and yellow respectively) you can see that the <body> element is contained within the <html> element.
It's possible, though, to squeeze a little more space (10 pixels on a retina display) out on the left by adjusting the left margin by 5 (CSS) pixels. For images, 5 pixels means 5 screen pixels as seen above.
On the right side, it turns out that setting "width:100%" on the body element, which you'd expect would be innocuous, actually pushes the the right side a little beyond the <html> edge, so things get clipped.
So presumably replacing "100%" with the appropriate pixel value would get you exactly to the edge...