It seems to me that Dutch is closer to German in its pronunciation than in its writing. It is a bit approximative to say so, and that's rather a personal impression, shared by some German native speaker and non-Dutch speaker I could talk to.
For example, "einfach" and "eenvoudig" are quite dissimilar in writing. However, when pronounced, the proximity is obvious. We could also mention "verduidelijk" and "verdeutlichen" or "Waarden" and "Werte".
In this regard, speaking myself German and learning / reading Dutch, I have the impression that Dutch is spoken dialect of German with a different writing (a different phonetics).
What in the history of those languages could explain a greater proximity in the pronunciation than in writing for German and Dutch?
Did Dutch derive from German or a common ancestor more similar to German, so that it could be thought as a "deformation" or a "dialect" of German?
What are some reference books, web sources, or experts on this question?