This series is difficult to track, as the series numbers are not readily available in most photos of the covers, advertisements, or PW listings. WorldCat has a few numbers, and I've seen a Catalog of Copyright Entries listing for #151 (Painting in Our Times by Grosser). The lack of a prefix makes it difficult to obtain hits in databases, so I may leave this series well enough alone beyond providing basic information about it.
Bobbs-Merrill Enters the Paperback Field, PW, Apr. 16, 1962, at 74 (announcing Charter Books, which were taken from the B-M trade and law backlists, the technical backlist of Howard W. Sams & Co., and the Liberal Arts Press, another Sams subsidiary.)
Ad, PW, Apr. 16, 1962, at 30 (announcing Charter Books as available on Aug. 25, 1962, and listing the first 20 titles in the series; also includes their pitch):
an advanced publishing concept, making available best-selling books of proved literary merit and wide general appeal, selected to fill the present gap between mass-market paperbacks and the highly esoteric book lines . . .
Charter Books Reports 100,000 Advance Sale, PW, July 30, 1962, at 44-45 (including photo of 3 display racks of Charter Books on p. 45).
Ad, PW, Aug. 27, 1962, at 26-27 (two-page spread that includes images of some of the titles).
Ad, PW, Sept. 17, 1962, at 26 (similar ad with bigger images of some of the titles).
Ad, PW, Oct. 8, 1962, at 11 (assertive ad -- "HALT THE ROVING EYE" -- that touts their manufacturer, American Book-Stratford Press, Inc.)
Photo, PW, Nov. 12, 1962, at 82 (image of a "combination pegboard or counter-display unit" with Charter Books in it).
Ad, PW, Dec. 17, 1962, at 6 (claiming that one of Bobbs-Merrill's 1962 highlights is Charter Books "mak[ing] paperback publishing history.")
Ad, PW, Aug. 19, 1963, at  (listing the Fall 1963 Charter Books titles and noting there are 44 titles available).