@mrundkvist
More like 30 sec once you know the secret. But which file? Reading on what reader? Anyway, for the epub-3 version, open in the #Calibre editor and you find in the 1.css file:
body {
margin-left: 20%;
margin-right: 20%;
text-align: justify;
}
Just change the margins to something like 2% instead of 20.

You might also do an epub-->epub conversion and mess with margin settings, might work, I prefer to edit.

@ebooks #ebookediting