The bikeline books Rhine Radweg series and Donau cover the whole route. You can get them in English or German. Although if you buy them in Germany, you may struggle to find the English ones. We rode the Rhine and Danube through Germany last year. The books are manageable if you can only find the German ones. Be warned though they don't allow for much distance either side of the river, so if you plan to make side trips you will need other maps.
Have fun
There is some information on our blog if you are interested.
ontheroadwithatoad.blogspot.com.