Have you ever bought a book by judging its cover? Yes, we all have.
The book cover is an important part of a book, it attracts readers. 
The first books had wooden covers. Some 17th Century books had papers stuck together and used as covers. Wood and paper covers were then exchanged for the cheaper Millboard. Millboard is made from the fibres of old tarred ropes. It’s very strong and tough.
Today, we have a ‘one cover fits all’ policy but previously book covers were custom made.