"Reader's Digest Condensed Books" is a series of books published by Reader's Digest, running from 1950 until the present. In 1993, it was renamed to "Reader's Digest Select Edition", but the concept and the format has been essentially constant since 1950. The idea is to take the basic idea of Reader's Digest, collecting and condensing magazine articles, and apply it to books. Several times a year, four popular books were edited down to a more managable length and collected together into one hardbound volume of a a few hundred pages. They were sold by subscription. Most of the works were current and popular, with some of the titles condensed being books we would still read today, while others were popular spy or romance novels.
Having explained the objective facts of the Reader's Digest Condensed Books series, let me throw in an opinion, although it seems to be a common opinion: the idea of the series was rather unnecessary to begin with. Have you ever thought "I wish I could read a John Grisham novel, but I would like to do it in one-third the time, so I can then read a shortened romance novel"? Most of the books condensed were light reading that people would be reading to kill time anyway, so the idea of shortening them seems silly.
Reader's Digest had a long reach, and these books were marketed well, so millions and millions of them exist on bookshelves, or more likely, in cardboard boxes in attics. People have an aversion to destroying books, which means that these old books persist, peering out at us from dusty bookshelves. Due to the glut in supply, they have no real value to collectors. Even at the library book sale, we didn't attempt to sell them but used them as structural elements to prop up tables. There are just enough real books in circulation that there is no real demand for a haphazard collection of four shortened popular works put together. But these things will probably keep turning up from spare rooms and storage spaces for the next several decades, especially since they are still being produced.