Returning a partial nested document in ElasticSearch

I'd like to search an array of nested documents and return only those that fit a specific criteria.

An example mapping would be:

{"book":
    {"properties":
        {
         "title":{"type":"string"},
         "chapters":{
                     "type":"nested",
                     "properties":{"title":{"type":"string"},
                                   "length":{"type":"long"}}
                                  }
                     }
          }
     }
}

So, say I want to look for chapters titled "epilogue". Not all the books have such a chapter, but If I use a nested query I'd get, as a result, all the chapters in a book that has such a chapter. While all I'm interested is the chapters themselves that have such a title.

I'm mainly concerned about i/o and net traffic since there might be a lot of chapters.

Also, is there a way of retrieving ONLY the nested document, without the containing doc?