The first bookmobile was established in 1905 at the Washington County Public Library (Md.). The original bookmobile was a wagon with horses which were driven by the library janitor. The was wagon had room for 200 books. Mary L. Titcomb was the librarian who instigated this innovation. It was originally designed for service to children. The bookmobile continues today with an emphasis on seniors and children though the Washington County website states that it will provide service to anyone who cannot access the library through other means.
The website also contains a video showing great historical images of the bookmobile and discussing its current services.
The Library History Buff blog has a bookmobile tribute which includes images of the Washington County Public Library's first motorized bookmobile. It also shows a great postcard collection depicting bookmobiles from around the country.