So I'm expecting any successful civilisation to start colonising the spaces between the stars, rather than the star systems themselves.
Before you start writing your book, you might want to familiarize yourself with the prior art in the field: start with the short story "Swarm" by Bruce Sterling, then the novel "Permanence" by Karl Schroeder, "Blindsight" by Peter Watts, and maybe "Diaspora" by Greg Egan. That's assuming that by "planning a book" you're thinking in terms of fiction rather than SETI-oriented non-fic, but even if you're writing non-fic it's worth noting that we SF authors have been all over the topic for decades. (As for non-fic, the prior art goes all the way back to J. D. Bernal's "The World, The Flesh, and The Devil", if not further: I don't read Russian but I gather Nikolai Federov wrote on the subject, as did Konstantin Tsiolkovsky.)
I've read all of those! (I actually discovered Karl Schroeder from your blog; for which I thank you. I gather _Lockstep_ addresses the same subject, but haven't read it yet.) Although they're very much focused on going substellar objects, e.g. brown dwarves or dark giants, while I'm thinking of the vastly more common asteroidal bodies. It's not a subject I've seen addressed very often.
Here's the paper that crystallised the idea for me:
Before you start writing your book, you might want to familiarize yourself with the prior art in the field: start with the short story "Swarm" by Bruce Sterling, then the novel "Permanence" by Karl Schroeder, "Blindsight" by Peter Watts, and maybe "Diaspora" by Greg Egan. That's assuming that by "planning a book" you're thinking in terms of fiction rather than SETI-oriented non-fic, but even if you're writing non-fic it's worth noting that we SF authors have been all over the topic for decades. (As for non-fic, the prior art goes all the way back to J. D. Bernal's "The World, The Flesh, and The Devil", if not further: I don't read Russian but I gather Nikolai Federov wrote on the subject, as did Konstantin Tsiolkovsky.)