[GeoffR]

The !important on the properties of the book-columns and book-inner divs is probably not necessary in most cases, except that if there was something like div {margin-left: 1em !important} in the book's stylesheet then that margin would be applied to those added divs too, which would change the book in an unexpected way (since the author has no way to know that the driver is adding extra divs.) However that is probably not very common, so it would be OK to leave it out.

The current firmware patch doesn't use !important when adding padding to these divs because there is currently nothing to override, but I can change it so that the patch uses !important.

The order of priority goes like this as I understand it (lowest to highest priority):
1. device stylesheet
2. book stylesheet
3. book stylesheet !important
4. device stylesheet !important

So without !important the device stylesheet just provides a default which is overriden by the book stylesheet, but if the device stylesheet uses !important then it overrides everything.

Edit: Allowing for the style-hacks.css (I assume this is added at the start of the book's stylesheet?) and kobo_extra.css (added at the end of the book's stylesheet):
1. device stylesheet
2. style-hacks.css
3. book stylesheet
4. kobo_extra.css
5. style-hacks.css !important
6. book stylesheet !important
7. kobo_extra.css !important
8. device stylesheet !important