I've done EV6 (along the Danube) from Donaueschingen as far as Bratislava and for most of the way it's either on car-free paths, bike paths alongside roads, or quiet back roads. EV15 (Rhine route) between Koblenz and Konstanz is much the same. Signposting on both was good; you might argue that it's hard to get lost following a river but you don't stick to the riverbank all the time and go through various towns and cities as well as crossing over at some places. I've followed part of EV13 (Iron Curtain route) around the Austrian/Czech/Slovak border which never goes near a major road, because there aren't any along those borders. I have a vague plan to do more of EV13, probably the section through Germany.
The Bikeline books are good if they cover the parts you're interested in.
E.g. parts of the North Sea route through Netherlands/Germany/
Yeah, that looks flat.