This shortcode was added in version 3.2.3.
[purchase-links] will display the purchase links added to the current post. You can display the links for a sub-book by passing the book number in as a paramter (similar to [book-info]). Example:
[purchase-links]- Displays links for the main book
[purchase-links number="1"]- Displays links for the first sub-book.
[purchase-links number="2"]- Displays links for the second sub-book.