Books created in the book module plugin in versions prior to Moodle 2.3 don't remain intact after a site or course has been converted to 2.3. Here is a suggested method for retrieving and restoring book content in a 2.3 course, assuming that both the 1.9 and 2.3 versions of the course are accessible:
1. Create a backup file of just the book content from the 1.9 course
2. Put the backup file in a file repository
3. Restore the file into the appropriate 2.3 course, merging the content
4. Move the book resources in the outline if necessary
You could create all of the book backup files prior to upgrading the Moodle site if you don't intend to maintain the 1.9 site separately.