Hougthon & Mifflin's Riverside Editions, a series of reprints of classics for the school market whose name harkens back to the company's 19th century Riverside Editions series, were brought out in 1956. The numbers can be confusing at first glance, as it's not clearly explained what the three subseries are (A - American, B - British, and C - Continental) or that they all share the same numbering scheme. In this one series the numbers run continuously, for example, from A58 to B59 to C60.
Houghton to Launch Series of Paperback Classics, PW, Apr. 14, 1956, at 1725-26.
Houghton Mifflin . . ., PW, Apr. 28, 1956, at 1889 (clarifying that the series is available at educational rather than trade discounts as the initial article stated) .
16 Best Sellers 60 (May 1, 1956):
Houghton-Mifflin have entered the paper-bound reprint field with their new Riverside Editions, similar to the Rinehart Editions in format and price. We have seen the following and find them good, particularly for use in college courses where reading complete works is preferred to the anthology: Pride and Prejudice, by Jane Austen, (B-1, $1.20; I); Middlemarch, by George Eliot, (B-6, $1.20; I), The Portrait of a Lady, by Henry James, (A-7, 0.80; IIa), Selected Writings of E. A. Poe, (A-11, $0.95; I); and Newman's Apologia Pro Sua Vita, (B-10, $1.05; I). All are beautifully printed and edited.