simplify book_index example reducers

The output of Hadoop reducers are never sent back to
reducers. Reducers are only called once per key. I first learned
MapReduce in the context of MongoDB, which reduces and re-reduces data.
Hadoop instead uses combiners for this intermediate "reduction". See
http://stackoverflow.com/questions/12956892/is-wikipedias-explanation-of-map-reduces-reduce-incorrect/12957119#12957119
for a decent explanation.