It now seems clear that my next book project will be on the Dutch Stonewall. No contracts or written agreements are yet in place, so details must wait, but it seems that my co-author will be one of the real experts.
The Stonewall was the height of fashion in the late eighties and early nineties. It featured in the repertoire of many top players, including Kramnik, Short, Ivanchuk, Bareev, Jussupow, Dolmatov and Agdestein. The main discovery was that Black could develop his bishop to d6, rather than to e7 as Botwinnik had done in the fifties.
This position, normally arising from the move-order 1.d4 e6 2.Nf3 f5 3.g3 Nf6 4.Bg2 d5 5.0-0 Bd6 6.c4 c6, became the main focus of the debate:
Black has weaknesses but he also has a solid position, space and an easy plan for development. Quite frequently Black will be able to generate a kingside attack.
Obviously White too has his chances. They may even be slightly preferable but that's the nature of chess.
White's two main continuations are 7.b3, planning to exchange dark-squared bishops with Ba3, and 7.Bf4, which achieves an immediate exchange of bishops at the price of a slight weakening of his kingside.
It's not clear why the Stonewall's popularity slowly declined. To some extent it may have been a question of fashion's whims. Also many of the original aficionados retired from competitive chess or at least became less active on the tournament scene. But this doesn't seem to completely explain the development.
For the moment my theory is that the Dutch Stonewall declined in popularity because the French defence did the same. The Dutch defence (or rather counter-attack) is much easier to play if you can confidently meet 1.d4 with 1...e6, but that's only an option if you don't fear 2.e4. So one of the main goals of the book must be to show that 1.d4 f5 is a perfectly valid move-order.