Drupal 6 import/export book
The module Migrate is the most versatile Drupal tool for database migration.

However, book module migration is still not trivial.  Books are nodes, so they can inherit most of their migration from a common class.  But to preserve book node hierarchy that is stored within menu links, you need to use special code  to migrate the hierarchy.  Here is a sketch of the required workflow:


Migrate nodes as normal.
Get hierarchy from the source database book table, and weights from the menu_links table.
Place a public function complete() in your Migration class to call _book_update_outline() to migrate the hierarchy.


A more detailed receipe, with a special code example, is at Drupal.org: Book node migration.