This book is a young adult speculative fiction story following two characters, named Lily and Canna, who are both afflicted with a fictional disorder called "shifting." This disorder causes them to periodically teleport between one of two nearly identical universes, with the exact moment at which they move between realities always being random. The main plot of the novel revolves around a crisis which Lily and Canna both face when they learn that at some point in their immediate futures, they will "settle" in one of the two worlds they have so far lived their lives. In the process, they will permanently lose touch with the people whom they know in the other universe.
The complication on this setup however (and the aspect of this book which I think makes this novel a really interesting story to follow) is that Lily and Canna are also indicated by West to physically be the same person. That is, each of their identities are alternate manifestations of the other's personality, and emerge the instant they shift between realities. This sets up Twice Lived to be a very subtle psychological story examining how both of these characters have created barriers between the two lives they are simultaneously leading in either world. An excerpt from the review is below:
Due to the stresses involved in a life spent randomly shifting between worlds, both Lily and Canna have learned to carefully divide the expectations hoisted upon them by others into two distinct identities, both of which have not only their own family and friend groups, but also personal interests and hopes for their respective futures. When one character vanishes from the reality they call their home, the identity of their counterpart reappears in the other universe. Yet now that they are nearing the point in their lives at which they will likely settle, they are both coming to contemplate the simple realization that only one of the two lives they have so far lived will continue on into the future.
At least for me, one of the most fascinating aspects of Twice Lived was that as the novel's story develops, the division that Lily and Canna have constructed between the two halves of their identities slowly comes to also be reflected in the actions of the characters surrounding them. Ultimately, everyone in Lily and Canna's life is shown to have divided their lives into separate dimensions of experience, and as a result struggle to fully understand one another due to how they have willed themselves to ignore huge aspects of their lives.
The complete review which I wrote for Strange Horizons can be found here.