There is a separate map/guide for every segment of the North Sea
route. Some countries have more than one segment. I took the Dutch, German, Danish and both Norwegian guides. That is quite a lot of paperwork to carry and I sent stuff home after using it.
The guides come from different publishes, some are map books, some are foldout maps, others are map-route cards. The Norwegian cards are weatherpoof.
I bought mine from Sustrans.
The other guide you need is Overnatning in der Frei, a guide to free and low cost campsites in Denmark, which you can buy in larger tourist info offices such as Tonder just over the German border. When people say that Denmark is expensive, they don't have that guide.
The route is mostly well marked and some people manage without the guides but I found them really useful with stuff like campsites and alt routes.
Check out the Dutch islands such as Texel and Vlieland and the Norwegian island of Kvitsoy.
Do the route anti-clockwise from Holland to Norway, the route gets harder and more scenic as you travel.
My one regret is leaving Denmark with enough Danish Kroner in my pocket to buy coffee and cakes.