Not "a dozen more," but in the book world used as an abbreviation for a duodecimo, a small book (a little smaller than a mass market paperback), whose printed paper sheets were folded in such a fashion as to make twelve leaves (24 little pages) from one large sheet. And yes, it is sometimes pronounced "twelve-mo." Beatrix Potter's famous Peter Rabbit books, such as this nice example from our Catalog 84, were printed as 12mo's.