Does anyone know whether there is a grammar section in Duolingo? It not, could you please recommend a good book for basic grammar in Dutch?
I don’t know of a collective section, but on the desktop website if you click on a topic (like when you’re about to practice), there’s a light bulb, which will redirect you to a grammar page. It’s got some good explanations, however, you might need to Google for more in-depth material (or use Jersebas’s links).
Duolingo's (mobile) web version (www.duolingo.com) has Grammar "Tips and notes" in a lot of the lessons that introduce new grammar:
Tap the "light bulb" icon next to the "Start" button
Extra for Dutch:
Overview of grammar explanations
On the bottom of that webpage is a link to
Helpful external material