Butterfly binding is Chinese traditional book making method. It is similar as many accordion flyers connected together.
According to Wikipedia, the butterfly binding was invented during the Song Dynasty. A printed paper sheet was folded in a stack and pasted together, plus front cover and back cover, punch holes along 1cm book spine and thread stitch.
In recently years, many people pursue traditional Chinese culture. Many Chinese publishers publish this type of books. They choose 40gsm light offset paper and bind by hand. Many butterfly binding books are limited edition. More fans collect many this type of books. For example, Chinese B/W picture-story books and some ancient poetry books.