This consists of a package containing 2 CDs plus a hardcover booklet (130pp) of extra notes.  The CDs hold good quality recordings of bird song; unfortunately, it is sometimes difficult to disentangle individual calls when several different birds or species are talking at once.  The notes are necessarily brief and don't always inlude information on habitat, which to my mind is a serious omission in something advertised as a "field" guide.  Not for beginner twitchers, I would say, but interesting and informative to listen to.