A Publishers Weekly article on the impact of online references like Wikipedia on reference publishing — multi-volume encyclopedias are essentially toast — has the following passage about maps and atlases:

Encyclopedias aren’t the only place publishers are feeling pain, though. At the National Geographic Society, head of book publishing Nina Hoffman says cartography is another category hit hard by the Internet. “Maps and pocket atlases have been severely hurt,” she says.

But Hoffman emphasizes that more in-depth offerings still do well. “Big atlases haven’t been dramatically affected in the same way,” she says. In fact, National Geographic will release its first $100 atlas, The National Geographic Visual Atlas of the World, in October.