Library of Liberal Arts
It appears that the original title of the series was The Little Library of Liberal Arts. Initial books published in the series had series numbers that looked like “No. _”; later (after Bobbs-Merrill acquired it?) they had the series number form “LLA _”.
Liberal Arts Press Merges With Bobbs-Merrill, PW, Jan. 16, 1961, at 97-98 (announcing merger and noting that the found of the Liberal Arts Press, Oskar Piest, will become a vice-president of Bobbs-Merrill and the general editor of the new Liberal Arts Press Division of Bobbs-Merrill).
Ad, PW, Jan. 16, 1961, at 52 (announcing merger between Liberal Arts Press and Bobbs-Merrill).
There’s one title that has the number LLA s 1 (Ducasse - Art, the Critics and You) (see Ad, Publishers Weekly, June 8, 1964, at 48.)