From a NLS Blog Post:
In the past few days [the Maps Reading Room team has] put a 3D map viewer online. We think you’ll like it.
It provides the opportunity to explore the map collection from a bird’s eye perspective, swooping down over towns, cities and mountains to view maps in a way you may never have viewed them before. You can alter your altitude, tilt and orientation to explore any one of our 600 georeferenced map layers draped over a 3D landscape. It is also possible to fade the transparency and view different base maps.