code: function listAssets(book, pages) {
      var fs = book.getContentFS();
  
      var summary = book.getSummary();
      var summaryFile = summary.getFile().getPath();
  
      var glossary = book.getGlossary();
      var glossaryFile = glossary.getFile().getPath();
  
      var langs = book.getLanguages();
      var langsFile = langs.getFile().getPath();
  
      var config = book.getConfig();
      var configFile = config.getFile().getPath();
  
      function filterFile(file) {
          return !(
              file === summaryFile ||
              file === glossaryFile ||
              file === langsFile ||
              file === configFile ||
              book.isContentFileIgnored(file) ||
              pages.has(file)
          );
      }
  
      return timing.measure(
          'parse.listAssets',
          fs.listAllFiles('.', filterFile)
      );
  }
docs: List all assets in a book
  Assets are file not ignored and not a page
  
  @param {Book} book
  @param {List<String>} pages
  @param