Books are just collections of pages divided into chapters and sub-chapters.
Book Resources are a little bit more complicated to create that Page Resources. Like some other Moodle activities/resources you have to initially create the Book without any content and then go into it to start creating each page. So it is a multi-stage process. The reason for this should be obvious, if you are creating a book with 20 separate pages, you do not want to have to enter all the data for every page in one go. So you create it a page at a time and can then rearrange the pages if you wish. Here is a video on how to do this.