Returning to the initial question about 2. Bg5. Summerscale's book deals with 2. Bg5 from a white point of view (as alumbrado writes), while a number of black repertoire books see it from the other side, among these are
Understanding the Leningrad Dutch by Beim,
Play the Classical Dutch by Williams,
Classical Dutch by Pinski and
The Dutch for the Attacking Player by Pedersen (a Leningrad book). I don't know the other sources as e.g. the new Starting out... or Bellin's older but highly acclaimed book.
Here's brief confrontation between Summerscale and the black repertoire books mentioned.
Beim, Williams and Pinsky all recommend 2.- h6, and the critical variation seems to be 3. Bh4 g5. 4. e4 (the move preferred by Summerscale, 4. e3 and 4. Bg3 do not give black serious problems according to all sources) Nf6. 5. e5 e6. 6. exf6 Qxf6. 7. Bg4 f4. 8. Bd3 (this is not mentioned by Summerscale who plays 8. Nf3 Nc6. 9. c3 b6. 10. Bb5 Bb7. 11. Bxc6 dxc6. 12. Nbd2 with an edge for white, but Williams and Pinsky deviate with 9.- d6. 10. Nbd2 Bd7. 11. Ne4 Qe7. 12. Bxf4 gxf4 with an equal game for black) Nc6. 9. Qh5+ Kd8. 10 c3 d5. 11. Nf3 Bd7. 12. 0-0 Bd6. 13. Nbd2 Ke7. 14. Rae1 Rag8. Pinsky thinks black is OK here, while Williams gives white the advantage after 15. Nb3. I think Williams is closest to the truth, but the position is very unclear.
Pedersen recommends 2.- g6 and the critical line is 3. Nd2 d5. 4. e4 fxe4. 5. Nxe4 d5. 6. Nc5 b6. 7. Nb3 Nf6. 8. Nf3 0-0. 9. Be2 Qd6 (in Pedersen's book, Summerscale only considers 9.- Ne4). 10. 0-0 Nbd7. 11. Qc1 e5. 12. dxe5 Nxe5. 13. Bf4 Nfg4 "gives rise to a level position" (Pedersen).
I think the positions arising from 2. Bg5 are generally quite dynamic and unusual. The line is fully playable and most likely leads to positions much different from what a Dutch player is used to. I have tried it a few times with mixed results (the first time I lost with white in 12 moves

, but that was not because of 2.- Bg5).
It could be interesting to hear which comments the Dutch experts of this forum have on the 2. Bg5 variation.