A book in InDesign is a file that contains links to multiple InDesign documents. You can make a new book by going up to the menus and selecting File -> New -> Book
The purpose of a book is to make it easy for multiple InDesign documents to be concatenated. For example, you could have five people working on one publication. Each person would have their own document. Person 1 could have page 1, Person 2 has page 2 and 3, person 3 has page 4 and 5, and so on. Then, when you make the PDF, have one of the computers connect to all the others, and make a book containing all of the InDesign Documents. Then, make your PDF from the book. And poof! You have one PDF with all eight pages.