I hope it is a good book. It is a difficult subject. I tried to build a double fianchetto repertoire for white starting with 1.Nf3, using some material from GM Damian Lemos. I put quite a lot of effort into it and played some OTB games, but I didn't enjoy the games as much as I usually do, sad to report. I've gone back to the Trompowski with 1.h4 for variety. No dull games with those (especially 1.h4!).

It’s out now! I‘ve just ordered at Niggemann and hope to get it tomorrow.

https://thinkerspublishing.com/product/daniel-hausrath-double-fianchetto-the-mod...

There‘s a free preview with 25 pages!
Seems to be a game collection with double fianchettoes from White and Black. Some theoretical notes, lots of comments on structures and move-orders.
White double fianchetto against King‘s Indian, Grünfeld, Dutch and Queens Gambit...
Black double fianchetto in the AccDragon, Caro-Kann, Modern, Hippo ...

tracke

As a promotional gift I got the 2020 calendar from Thinkers Publishing.
Contains some nice photos with a young lady holding chess books from TP.
And some covers of published or forthcoming chess books from TP, among them:

Double Fianchetto - The Modern Chess Lifestyle by GM Daniel Hausrath

Obviously this book is planned for 2020 but I haven’t found any other hint,
neither an advertising blurb nor a publication date.
But looks like an opening book. Or at least a book about middlegame structures
with strong relations to opening theory?!
[PS: The cover in the calendar shows a diagram from the left (a-file at the bottom)
after the moves b2-b3, g2-g3, b7-b6 and g7-g6]

Looking at the databases, I found that Hausrath has played some Hippos and
some Tigers but recently most often Modern Philidor and King’s Indian with black.
But as White Hausrath is a specialists for several Reti-English setups after 1.Nf3,
sometimes with 2.b3 but more often with 2.g3, followed by an Double Fianchetto!

tracke