Turn editing on
Click on the edit icon ( ) next to the book.
Book Administration appears under the Settings Block on the left side.
Click Import Chapter to bring up the Import Screen
Type options allow you to have multiple HTML files loaded as individual chapter, or each to folder in a ZIP to be regarded as individual Chapters.
Choose a file....to browse for uploading.
Otherwise drag & drop ZIP files.
Import once you have uploaded all ZIP files that you want imported.