[capink]

You have one of two approaches:

• One Formulas Action: You replace the formula you have above with this one:
  

  program:
      report = '';
      for book_id in from_selection('id'):
          report = strcat(
              report,
              character('newline'),
              book_field(book_id, 'author'), ' - ', book_field(book_id, 'title'), character('newline'),
              'Chapter Count: ', book_raw_field(book_id, '#chaptercount'), character('newline'),
              'Page Count: ', book_raw_field(book_id, '#pagecount'),character('newline'),
              'Average Pages per Chapter: ', format_number((divide(book_raw_field(book_id, '#pagecount'), book_raw_field(book_id, '#chaptercount'))), '{0:5.2f}'),
              )
      rof;
      report

  As you can see it uses for loop to traverse all selected books (from_selection function). It can access metadata for books using the book_field() and book_raw_field() functions, both of which needs the book_id.
  
  This might be slightly slower than the second approach, I've never done any bench-marking and not interested in doing it, because the speed is fine for me.
  
  
  
• Two Actions Chain (Chain Variables + Formulas): If you don't want to dabble in plugin specific functions like from_selection, book_field ...etc, do the following:
  
  First add a Chain Variables action, then add a variable called report and remember to tick the iterate checkbox, then add this template:
  
  

  program:
      report = globals(report);
      strcat(
          report,
          character('newline'),
          $author, ' - ', $title, character('newline'),
          'Chapter Count: ', $$#chaptercount, character('newline'),
          'Page Count: ', $$#pagecount,character('newline'),
          'Average Pages per Chapter: ', format_number((divide($$#pagecount, $$#chaptercount)), '{0:5.2f}')
          )

  Now add a Formulas Action with the following template:
  
  

  program:
      globals(report)

N.B This is the same approach I talked about when I told you to narrow down the books scope using templates scope. So if you have an action that needs to act on a subset of selected books, you filter the books ids you want to act upon using one of the two above techniques, and feed the list of book_ids to the action via template scopes.