Is there a Rick Steves guide book out there for river cities on the Danube, Main, and Rhine Rivers?
Not as such, but here are all the RS guidebooks (now on sale). You could probably assemble a collection that would cover most of your route.
When we did this route on a river cruise I bought the RS guides: Budapest, Amsterdam and the Netherlands and Germany. I think the guidebooks had something on nearly every stop as well as some cities/towns that were optional tours on the river cruise.