In my day, and in Minnesota, it was called the Arrow Book Club. The flyers came out monthly and my parents let me order as many as I could read (we had no TV in my house until I was 10, so I did a lot of reading). The paper backs were just wonderful--I can still remember so many of the ones I bought. They would range in price from .15 cents each to about .45 cents apiece. The classroom order would come in, in a great big box, and the teacher would distribute. I think every student got at least one or two--some were joke books or sports trivia, and then there were a lot of classics, but also fun and funky ones, too. It was a huge part of my 2nd, 3rd and 4th grade life.
Then we moved to Illinois for 5th grade, and I think they had the Scholastic Book Club there, so I continued it for 5th and 6th grade. Then we moved downstate to a university laboratory school for junior high--no book club anymore. Moved to the library then.
Books were a very big part of my childhood, and these book clubs, especially the Arrow Book Club, were so inexpensive that a single dollar could net you several books.