Multiple books
You might not want your website to take the form of a single, enourmous, all-encompassing book. You might want to create multiple books, for example if you are covering several unrelated topics on your site.
For example on cookiewiki.com, we might decide to add some articles about growing your own vegetables along with the recipes. These are related (people who grow vegetables are often also interested in cooking, and vice versa), but they don't really belong in the same book.
Adding extra books is exactly the same as adding your first book. On Cookie Wiki we could add a new book, “Grow your own”.
You new book will appear in the Book navigation block, just like the first book. Books are arranged in alphabetical order. To change the order of books in the Book navigation block, simply edit the top level page in each book, and set its weight to a suitable value so it appears in the right place. Unfortunately, there isn't a drag and drop way to reorder books … but usually writing a new book is a major undertaking, so spending an extra couple of minutes adjusting its position in the book list is no great problem.
One tip – when you add a new book, make sure the top level page is Published, otherwise you may get problems with some versions of Drupal.
Sites with lots of books
If you have lots of books on your site, the standard Book navigation block might not suit your needs. For one thing it might have too many entries to be usable. You might want to provide a better way to navigate around your books at the top level.
From the Navigation | Administrate | Site building | Blocks page you can also choose the configure link next to the Block navigation item. Under Book navigation block display change the selection to Show block only on book pages.
After you do this, you will find that the Book navigation block will only appear on pages which are part of a book, and will only show the contents of that one book. On a page from the “Recipes” book, the block would show the contents of the “Recipes” book, on a page from the “Grow your own” book it would show the contents of the “Grow your own” book. On other pages (eg the front page), the block isn't dispalyed at all.
Each book is a separate entity, and you can have lots of different books on your site without problems.
The only question is, how do users find the books in the first place, if the Book navigation block isn't visible on the main pages of the site? Well the answer is simple, you must provide links to your top level book pages in some other way.
You could create a menu which lists all your books, and links to the top level page of each book. If you want you can make this a multi-level menu so that books can be grouped by topic.
Alternatively, you could have a links page, which might just list links to each book, or could also include a short description of each book.
These are manual processes, new books have to be added by you rather than being added automatically, but in exchange you get much greater flexibility and control.
If you simply want a list of all your books, Drupal creates one automatically at /book, eg:
http://www.cookiewiki.com/book
Use your own URL of course. Don't bother looking at the Cookie Wiki book list, because that site doesn't actually use Books!
Considerations for creating multiple books
If your website covers multiple topics, things which you would not expect to appear within the same printed book, then there is a clear case for creating multiple books. Users don't want to wade through content which is irrelevant to whatever they are looking for.
For larger books, even if the content is all related, there are also practical considerations. A book menu is usually a side menu. These work well if you have 2, 3 or perhaps 4 levels of subheadings, after that the menu just gets too wide, or cramped.
Similarly, if a page has a large number of child pages, it can be hard to navigate. So reducing the number of menu levels by “flattening” several levels into one doesn't always help. You end up with a long thin menu rather than a short fat one.
Splitting your site into multiple books gives you the ability to structure your site at the very high level, but still have the benefit of automatic menu generation at the low level.