the construction "holy of holies" is a literal translation of a hebrew idiom which is intended to express a superlative. examples of similar constructions are "servant of servants" (gen 9:25), "sabbath of sabbaths" (ex 31:15), "god of gods" (deut 10:17), "vanity of vanities" (eccl 1:2), "song of songs" (song of songs 1:1), "king of kings" (ezra 7:12), etc.