Not sure when they switched from hard cover books to cheap cardboard cover. The publications don't have the same, fancy, expensive look that the previous hard cover ones did. I used to marvel over the nice hard cover books, and the colors of them. Also, sizes of the publications seem to have changed. There used to be two sizes: big and small. They don't seem to have a standard for this anymore.
I remember how I realized the durability of the hard cover books. We had a dog who decided to chew up a "United in Worship" book (at least I think that was the book). A corner of the hard cover was chewed away, but the book remained pretty much intact. With the hard covers gone, the durability of the books has gone down. Perhaps this is a way to sell more books, replacing the ones that have worn out quickly, plus reducing cost to make them?