Alright book collectors, this ones a toughie. Books before ISBN's were on everything are so tough already, but these are even worse because Houghton Mifflin also published Riverside Editions for schools, which are much easier to find, and don't have the printed cover. You can find these mostly on eBay and Etsy when they pop up. Look for the A, B, and C numbers, which were publushed by Houghton Mifflin Company, whereas the school editions were published by Houghton Mifflin School.