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I know there are sci fi books that are more complicated than this one, but this one hits about the edge of my complexity-meter where it’s worth trying to puzzle it out, but not so hard that I feel stupid and confused all the time. Like his last book, Crouch is taking on themes of multiple timelines. In this case a person building a device that allows you to... sort of... go back in time. Intended as a device to help her mother avoid the terrible downsides of Alzheimers, it instead turns into a whole bunch of new terriblenesses and is deals with the implications of all the other people whose timelines are affected, and who aren’t quite sure what is happening. Interesting to read and the main characters are likeable and make human decisions, errors and choices.