Briefly, the book talks a lot about 4, the first movement of 5, and 6. The idea is in all these cases that JSB allowed an instrument which was conventionally low down in a contemporary hierarchy (harpsichord, recorder and viola) to take a very leading role. And consequently the music is a sort of metaphor for challenging hierarchies -- the music is a metaphor for social equality.
As far as religion is concerned, he acknowledges that JSB, following Luther, had a very hierarchical view of both music and society in this world (he points to some margin notes in JSB's bible, the nature of JSBs disputes with his work colleagues.) But he thinks that JSB, again following Luther, thought that in paradise there are no hierarchies of people. So ultimately these concertos are a model of the social organisation of the next world, not a revolutionary document for this one.
There's a lot of material in the book about the unity of the concertos -- whether they make an integrated set -- which I've only skimmed through so far.
I've found it an interesting read and it has certainly stimulated me to go on a Brandenburg binge. Your posts have been very helpful, premont, so thanks once again for having posted. I find myself enjoying Egarr's 5th for example (and yes, even the cadenza. Do yo enjoy it more now that a couple of years ago?) And some of Kuijken's second set too -- 6 and of course 3 and 4. Spotify makes all these recordings so easy to access!