The book I'm reading now, published by Open Road Media, has the body margins set like this:
Code:
body
{
margin-left:1.5%;
margin-right:20px;
}
So if you happen to be reading on a device with a horizontal resolution of about 1333px then you'll get nice even 20px left/right margins, but if your device has a horizontal resolution of anything other than about 1333px, then one margin will be bigger than the other.