Publisher of the highest-quality paperback books in the world.
Dover is in the business of reprinting "classic" books in inexpensive paperbacks. Unlike most paperbacks, though, Dover books are designed to last a long time, and, in the words of a used book dealer I visited today, "they keep their value better than just about anything."
The books are printed on archival-quality acid-free paper, and, uniquely for paperbacks, the pages are sewn into the binding so they won't fall out. This publishing house is truly an eclectic one; according to the back covers of the books
"Each year Dover publishes over 200 books on fine art, music, crafts and needlework, antiques, languages, literature, children's books, chess, cookery, nature, anthropology, science, mathematics, and other areas."
They offer a wide variety of free catalogs as well.
Dover is probably the only publisher in the world to sell serious, technical books on mathematics and hard science for less than US $20, many of which had been out of print for years or were far more expensive in their original, hardbound editions. This is a tremendous advantage to students who want readable material to supplement their often error-ridden and incoherent "official" class texts. And as I mentioned before, they last nearly forever. I have an old Dover edition of Hermann Weyl's treatise on group theory in quantum mechanics; though undated, it must have been printed before 1963 since the publisher's address lacks a zip code.