...I talk about Jessica Shirvington's One Past Midnight:
Shirvington clearly put a lot of time and thought into the mechanics of the world. Every time I had a question—has Sabine ever tried to find her other self? Is she two people in one world, or living in two different timelines? What’s the best place to hide a key when you’ve been committed to a psych ward?—she answered it, and then proceeded to answer three more that even I, in my infinite nitpickery, hadn’t thought of.