To partly answer my own question now, having read two of the series...
The "Mushrooms" volume (No 1) is an excellent overview for a general naturalist or someone starting out studying fungi. It's quite a light and enjoyable read but inspires one to a take an interest in all fungi (and not just the more obvious agarics and boletes etc). The author writes well, as you'll know if you have read his "Chasing the Ghost", "Rainbow Dust" or "Britain's Rare Flowers".
The "Meadows" book (No 2) is very different in tone, more academic and I think written by an ecologist with ecologists in mind (or not really able to write for a a more general audience). It's a curate's egg, though: some sections of it make rather dry and dull reading, while others are more engaging.
Having only read two of the eight currently available volumes, I'm not sure yet of the intended audience. They are hardback only and the price tag is quite hefty (about £30). Maybe the series is meant to rival Collins' New Naturalists?
I was given a copy of "Mountain Flowers" (No 4) for Christmas, so that's next.