Sentry Editions

Houghton Mifflin to Enter Paperback Field, PW, Mar. 20, 1961, at 33. 

Houghton Mifflin Ad, 55 ALA BULL.. 514 (1961):

This fall we will introduce Sentry Editions, a new development in paperback books. In a format of vinyl cloth, designed to last a lifetime, Sentry Editions secure America’s heritage of literature and history. The first twelve distinguished titles will appear September 11.

Houghton Mifflin Starts Paperback Series, 73 BOOK PROD. 77 (1961):

In September this year, Houghton Mifflin Company will begin publishing many of its titles in an alternate library-sized, 5⅜ x 8¼, paperback formate [sic], which will not replace the usual binding but will be available for those who prefer it. This series, to be known as Sentry Editions, will feature some of Houghton Mifflin's most distinguished and permanent titles. The bookstore prices will range from $1.20 to $2.45.

Charles A. Madison, BOOK PUBLISHING IN AMERICA 445 (1966):

In 1961 Houghton Mifflin began to issue Sentry Editions—library paperbacks of its rich backlist, priced from $1.95 to $2.40.