code: function() {
              var book = this;
  
              var styles = book.config.get('styles');
  
              return _.reduce(styles, function(prev, filename, type) {
                  return prev.then(function() {
                      var extension = path.extname(filename).toLowerCase();
                      if (extension != '.sass' && extension != '.scss') return;
  
                      book.log.info.ln('compile sass file: ', filename);
  
                      // Temporary CSS file
                      var tmpfile = type+'-'+Date.now()+'.css';
  
                      // Replace config
                      book.config.set('styles.'+type, tmpfile);
  
                      return renderSASS(
                          book.resolve(filename),
                          path.resolve(book.options.output, tmpfile)
                      );
                  });
              }, Q());
          }
docs: Compile sass as CSS