Dover Publications, also known as Dover Books, is an American book publisher founded in 1941 by Hayward Cirker and his wife, Blanche. It primarily publishes reissues, books no longer published by their original publishers. These are often, but not always, books in the public domain. The original published editions may be scarce or historically significant. Dover republishes these books, making them available at a significantly reduced cost.
Source: Wikipedia, "Dover Publications", available under the CC-BY-SA License.