Publisher: Harper & Row
Related Series: American Heritage Paperbacks, Colophon Books, Harrow Books, Harper Perennial Classics, Harper Religion Department Books, Jewish Publication Society Series, Jubilee Books, Perennial Library, Perennial Library 4000 Series, Torchbooks, Torchbook Library Editions
As with other Harper & Row series from the mid-1960s, the letter appended as a suffix to the series number in 1965-1966 corresponds to the book's price:
F - $.95
G - $1.25 / $1.35
H - $1.45
J - $1.60
K - $1.75
L - $1.95
M - $2.25
N - $2.45
P - $2.75
Q - $2.95
T - $3.75
It appears that these letters were used only from mid-1965 through mid-1966, and subsequent printings of these titles under the same series number both drop the suffix and have a higher list price.
PW, Dec. 7, 1964, at 43:
CHAPELBOOKS is the title of a new line of religious paperbacks which will be launched by Harper & Row on January 27. Harper says the paperbacks in the new line will be originals and reprints, and that the series is designed to appeal to the broadest Protestant church market, including laymen, study classes, and prayer groups.
The first seven ChapelBooks will be: "A Short History of Christianity" by Kenneth Scott Latourette ($2.45); "On Being a Real Person" by Harry Emerson Fosdick ($1.50); "The Common Ventures of Life" by Elton Trueblood ($.95); "For We Have This Treasure" by Paul F. Scherer ($1.05); "The Lord's Supper" by Harold E. Fey ($1.50); "Through Gates of Splendor" by Elisabeth Elliot ($1.75); "More Than We Are" by Marguerite Harmon Bro ($1.50).
Lots of high quality photos here: http://www.shunammite.com/idea/?s=ChapelBooks
This book shows the first 7 titles in the series (CB1-5, 7, 101): https://archive.org/details/commonventuresof011960mbp/page/n5