failing_mr.js

db = require('riak-js').getClient()

db
  .add([['catalogs', 'politics']])
  .link({ bucket: 'books', keep: false })
  .map(function(value) {
    // ejsLog('/tmp/mapreduce', JSON.stringify(value));
    value = Riak.mapValuesJson(value)[0];
    return [
      {
        id: value.id,
        title: value.title,
        author: value.author
      }
    ];
  })
  .reduce(function(values) {
    return values.sort(function(a, b) { return a.title < b.title ? -1 : 1; });
  })
  .run()

Thank you Frank, It works!