I just bought a copy of Play The Classical Dutch By Williams and am not very impressed.
To give it a quick glimpse, I compared it to my other Dutch book, by Christiansen and Silman. (That book and The Classical Dutch by Bellin are both excellent but dated 1989 and 197? )
Here is one thing I came across right away.
I haven't really looked at this with a board yet.
1.d4 f5 2.g3 Nf6 3.Bg2 e6 4.Nf3 Be7 5.0-0 0-0 6.c4 d6 7.Nc3 a5 8.b3 Ne4 (?! by S & C/Only move by W)
9.Nxe4 fxe4 10.Ne1 d5 11.f3 dxc4 12.bxc4 e5 13.d5 exf3 14.Nxf3 Qe8 15.Bb2
"and white was clearly better - S & C"
Hort - Levy Praia de Rocha 1969
15. ... Nd7 16.Kh1 Qh5 -+ better for black - Williams
Hartston - Levy Praia da Rocha Z 1969
Someone is wrong.