This is going as part of chucking books I'm not going to read again. Adam Roberts seems a well-enough respected sci-fi author, nominated for and winning various awards. Maybe he's written some good fiction. I don't know, because after reading On I'm not reading anything else by him. It's crap.
It's set in a world that's a giant wall. Everyone lives on just the minimal horizontal spaces of this big wall. As the book's awful, I'm happy to spoiler it: It turns out this wall is just the earth, but something happened and gravity's gone sideways. Like the book.
That doesn't really matter to the plot, as little as I can remember it, though, since it's the adventures of some kid who lives on the wall. It's all a bit medieval-ish, which makes sense, because none of the physics in this book make any sense, and people didn't really do physics in medieval times. QED.
To be honest, I don't remember a huge amount about this book beyond the huge sense of disappointment reading it, except for the way the appendix rubs salt in the wound: A rubbish pseudo-scientific explanation of the gravity shift, made all the more offensive by using a lot of actual physics-y maths-y stuff, to explain something that is clearly physically BS. Just don't bother. It's not crap sci-fi, it's crap fantasy pretending to be crap sci-fi.