This book was not what I had been expecting! This is a largely-epistolary novella between two time-traveling, timeline-editing entities from opposing "agencies" who can not seem to refuse the intellectual battle they've developed in the form of physics-defying, ephemeral letters they leave each other across space and time. 
I had an audiobook copy and a physical copy, and I think without the aid of the audiobook to keep me engaged, I would have been in and out of this story a lot more. It made salient points about humanity and the things that might be the same across time "strands" in the braid of all timelines these two agencies seem intent on curating. Red and Blue's dance was interesting at first, then seemed to drag in act two before it picked back up near the finale. The prose was gorgeous, but at times felt obfuscatory just for the sake of being confusing. 
Overall, I enjoyed it but I don't think I'll pick it up again.