Instruct: Classify a given Amazon review as either positive or negative.
Query: Great reference for the Najdorf after 6.Bg5

John Nunn's newest reference work on the Najdorf covers all positions after 1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 a6 6.Bg5. This edition expands significantly on his previous Najdorf work published in the mid-80's. In the ever-changing world of the Najdorf, it is essential to have the latest reference for any player rated above 2000. If you want to play the 6.Bg5 lines, then you need this book. John Nunn sets the standard that others are measured by when it comes to opening books. His works are full of original analysis and useful tidbits. Sometimes he does profess to enjoy a certain line over another and is not very clear on why (example: the Nb3 line of the Poison Pawn variation of the Sicilian), but generally he backs everything up with convincing comments.