It was bound to happen. Banks is experimental in his writing and sometimes those experiments resonate with me, other times they backfire.
This was a resounding backfire.
The basic idea is there is a secret society (in most parts of the multiverse) that employs gifted people able to “transition” between parallel realities, changing things as they go. The society (The Concern) is run by a sketchy cabal of odd folks, most notably a nymphomaniac, who have their own school to train these gifted individuals that’s a mix of Hogwart’s and the CIA.
It comes across as half-hearted. Many plot lines are irrelevant or superceded, like ideas that were abandoned but not edited out.
Just skip it, oh, and despite what Amazon says, it’s not a Culture book.