Appears to have been an offshoot of the Midland Books line, in so far as it was also a quality paperbacks line.
The Poetry Paperbacks have an odd publishing history, in that they were selective reissues of existing Indiana hardcover poetry books in paperback with their series numbers reflecting not paperback publishing order but the original cloth publishing order. To wit, one of the final Indiana Poetry Paperbacks, PPB 2 (The Gypsy Ballads Of Garcia Lorca), issued in 1964, was one of the first published poetry volumes in hardcover.