From the Smithsonian blog:

“This is a bookmobile—a library on wheels. These mobile libraries used to travel around bringing books to the people. The blog Messy Nessy Chic has a great roundup of images of these vehicles of bibliographic wonder. A traveling library often used to provide books to villages and city suburbs that had no library buildings, the bookmobile went from a simple horse-drawn cart in the 19th century to large customised vehicles that became part of American culture and reached their height of popularity in the mid-twentieth century.”

There’s also a Flickr site for bookmobile pictures from around the world.