Caplan began writing no later than 1920.[3] In the early 1920s he was editor of the Boston newspaper The Jewish Leader, which was published in both English and Yiddish.[4][5] Caplan edited the New Palestine magazine in 1934.[2] From 1940 to his retirement, in 1966, Caplan was editor of the Congress Weekly magazine.[6][2] After retiring, he was elected as member "at-large".[7]
Caplan, with Harold U. Ribalow, was an editor of the book The Great Jewish Books and Their Influence on History (1952).[11][12][13]
Caplan was married to Hannah Caplan (née Levin) and had two sons, Joseph and Leonard.[14] He died on May 6, 1969, in Woodmere, Long Island, New York.[6][1]
^The Maccabæan-Magazine of Jewish Life and Letters. Federation of American Zionists. 1920. p.82, p.94 (including about the aftermath of the Lwów pogrom (1918))