For pre-trip planning, www.holland.com is a useful resource.
Within the Netherlands, VVV (Vereniging voor Vreemdelingenverkeer, Netherlands Tourism Board; www.vvv.nl) is the official network of tourist offices around the country. Each tourist office is locally run and has local and regional info. Few VVV publications are free but tourist offices are a good place to buy maps and guides.
ANWB (Dutch Automobile Association; www.anwb.nl) has maps and guidebooks for sale. It provides a wide range of useful information and assistance if you're travelling with any type of vehicle (car, bicycle, motorcycle, yacht etc). In some cities the VVV and ANWB share offices. You'll have to show proof of membership of your home automobile club to get free maps or discounts.