The letters that make up about half of this book are gorgeously written, and I love the story they tell. The basic idea of the time war is clever, and the descriptions of placetimes the characters find themselves in evocative, sometimes reminiscent of Calvino's Invisible Cities. I devoured this book in a few days.
And yet... something about it felt a little thin or hollow behind its fireworks. I think it was a good artistic choice to leave all technical details out, but I couldn't help but get hung up on the time paradoxes. Not that it's the authors' responsibility to necessarily avoid or solve them, but for me personally they intruded on the suspension of disbelief.