Creating a book starts by creating a “Book page,” then adding additional pages to it. In order to create book pages, you generally need to ask for “book” privileges. Only then will you see some of the book creation options, such as the “Outline” tab, or “Book page” type. With proper permissions, different people may work collaboratively on a book.
The best practice is to use the “Book page” type when creating new pages in a book. The easiest way to do this is to use the “add child page” link. You’ll find one at the bottom of every book page. Clicking on it creates a new “Book page,” with the current page automatically selected as its parent.
Non-book pages (i.e. already existing pages in the system) may be added to a book. So non-book pages you posted previously, which you hadn’t intended to be part of a book, for example, may easily be added without the need to recreate the page as a “Book page” type. Because these pages lack the “Parent” and “Weight” parameters (necessary for positioning the page within a book) in their creation/edit dialogs, those parameters are made available under an “Outline” tab for the page (you will see the “Outline” tab alongside the “View” and “Edit” tabs if you are the page’s author and have book privileges).
Note: The “Outline” tab is a bit of a kludge to retrofit the book structure onto non-book page types, something Drupal did not originally support. You will not see it on a “Book page” because you can choose a parent when you create (or edit) it. Since other page types lack this option on their creation/edit pages, they require the “Outline” tab to retrofit that functionality. This is a subtle difference between a “Book page” and a “Page” type.
Strictly speaking, new books are only created when a page is assigned “<top level>” as its parent value. Only system administrators can do that. However a book may be effectively started at any level by anyone with book privileges, and later moved by an admin to become a truly standalone book. Technically, such books begin their lives as chapters, and are later promoted to book status. The Community Webmasters group on our site provides a place where you can create a book prototype which is generally out-of-sight of the public (and search engines), until you are ready to take the book live. All that needs to occur is for the top page of your book prototype to be moved elsewhere. Because all the links are relative, your whole book – and all its descendants – can therefore be moved by one simple page edit. You can use this approach with whole books, chapters, sections, or single pages.