Amy,
The only "sushi cookbook" I've used is Seductions of Rice.
http://www.powells.com/biblio/2-9781579652340-3The section there on sushi is quite small, but it explains how to make the rice quite explicitly ... and rice is what sushi is all about.
Since Kris isn't keen on raw fish, I've been uninterested in finding anything which has recipes for combination rolls.