Dover Thrift Editions is an imprint of the Dover Publishing Company, a publishing house dealing with out of print materials. The Thrift Editions are a series of classic works, which felicitously means that they are also in the public domain, sold at a very low price. The prices were originally 1 or 2 dollars, but they currently are still in the very reasonable range of under five dollars.
While reprinting public domain material is not something that only Dover does, Dover is notable for its no frills approach to it. While other companies will usually get an editor to write a foreward and put footnotes in the text, and may include a fancy cover and illustrations, and then copyright their edition, Dover Thrift Editions are bare bones, usually just the original text, on plain paper, with a simple cover. And, as the cliche goes, they pass the savings on to you, with all of the extraneous costs of publishing cut out.
And in most cases, these books are thrifty rather than just cheap. Most of these books are classics that don't need any decoration to make them readable. I think it is a great service to publish short, yet important works such as Common Sense and sell them for a dollar or two.
The only thing that can be said against the whole venture is that most of the works are so widely known and have been printed so many times that a new, cheap edition of them isn't really needed. After all, I would be hard pressed to go into a thrift store and not find a used copy of Huckleberry Finn for a very cheap price. However, the Dover Thrift Edition's extensive collection is still an easy way to get any classic work of literature for cheap.
Their current catalog can be found at:
http://store.doverpublications.com/by-subject-literature-dover-thrift-editions.html