Sadly, without more context or research, I can't tell you who the author of this 'the sleepwalkers broch novel' is. It could be a debut novel by a new writer trying to make a mark in the literary world, or it could be an old, forgotten piece of literature that only a few people are aware of.
Since I'm not familiar with this specific novel, it could potentially be a story that uses sleepwalking as a metaphor. Maybe it's about people who seem to be going through life in a daze, like sleepwalkers, and then something extraordinary happens to wake them up or change their paths.
I'm not sure specifically as I haven't read it. But generally, it could be about the inner turmoil or subconscious exploration of the characters, given the term'sleepwalkers' which might imply a state of being not fully in control or aware.
I don't know as I haven't read this novel. But typically, the main characters could be the sleepwalkers themselves. They would be the ones driving the story forward, with their actions and experiences during their sleepwalking episodes being central to the plot.
Sure, the Noragami manga is completed. It was a great ride with an ending that wrapped up the plot nicely. If you haven't read it yet, it's worth giving a go.