A predecessor softcover Houghton Mifflin series by the same name was begun in 1883, "intended for the use of schools and for general reading" (ad, PW, May 5, 1883, at [16].) This series was very successful, totaling over 300 volumes by 1925 (ad, PW, July 25, 1925, at [5]). Evidently the original series ended sometime in the late 1920s, but the name was revived for the quality paperback boom, alongside its new Sentry Editions, in 1962.
"Sentry" Cover Used on New Riverside Series, 76 BOOK PROD. MAG. 32 (1962)
The cover material developed for the Houghton Mifflin's Sentry Editions is now being used for its new Riverside Literature Series. The vinyl-coated cloth, Oznaburg [sic], furnished by A. D. Smith & Co., is being offset in four non-process inks, lacquered and bound on the Flexibound body of the book and then trimmed.