When it comes to byline, Banks had an interesting methodology: he published "fiction" as Iain Banks and "science fiction" as Iain M. Banks. Transition, published in 2009 in the UK, was by-lined as Iain Banks. Does that provide some insight as to how Banks himself regarded the story? What's even more interesting is that in the United States, the "M" was added. I suppose only the marketing execs could unravel that one for us.
Transitions is about an infinite number of realities traveled by individuals who are able to move their consciousness from one reality to another to inhabit different bodies by the use of a drug called septus. These individuals, called Transitioners, work for/are under the purview of an organization known as the Concern. The Concern sends Transitioners between realities to alter the outcomes of key events, thus guiding, in a supposedly positive direction, that reality's development.
Some of these missions involve someone dying, and that's where Temudjin Oh comes in. He's an assassin for the Concern, traveling from reality to reality and taking out bad actors and preventing bad futures. Temudjin is so good at his job that he gets noticed by Madame d'Ortolan, head of the Concern Council. Seems d'Ortolan has started to develop her own ideas about how Transitioners should be used and the purpose of the Concern.
These ideas happen to put her at odds with some of the other council members and higher ranking Concern operatives, in particular one Mrs Mulverhill, who leads a resistance movement against d'Ortolan. Mulverhill and Tedmujin have a past relationship and she wants him on her side. With d'Ortolan's actions seeming less and less on the up and up, Tedmujin is beginning to have his doubts about the Concern and its Council and the true nature of all the missions he's been on over the years. Ultimately he will have to decide which side he's on.
That's the plot in a nutshell, but there are a lot of other things going on as well, things (and characters) that can be a bit hard to keep track of since the protagonists are constantly hopping from one reality and body to the next among a web of convoluted intrigue.
So while Transitions might have originally lacked the the "M", I think it's pretty firmly in the realm of speculative fiction. Banks has delivered a novel full of sex, torture, and navel gazing philosophy that takes place in multiple realities, and is rife with court politics, backstabbing and assassinations. I seemed to me to be a political novel, exploring the nature of power and those who hold it (and who want to expand or keep it).
I'll confess I often lost track of what was going on. Maybe Banks was being deliberately obtuse, or maybe this is a novel beyond my meager ability to comprehend. Probably the latter. I did find it well-written and thought provoking, even that I was entertained even though I was't always quite sure what was happening....