35 year old Nora is miserable, her cat just died, and she decides that she sucks at living so maybe she should stop trying. However, instead of dying from the overdose she finds herself in a weird space between life and death that is a library containing an infinite number of books, each one the story of her life if she had made just one different decision. Would she have been happier if she hadn't bailed on her wedding at the last minute, or stuck with the band just as they were getting big, or studied science?
Obviously, parallel universes are a not uncommon trope in speculative fiction, but Haig sort of ignores all the quantum complications to focus on the central story. Nora can jump into any of her other timelines, and amusingly she remembers the life she is trying to leave but drops into the other possible timelines with no backstory, which makes things awkward when she finds herself married to somebody she doesn't know, or the lead singer of a world famous band without knowing any of the songs. At any time she isn't happy she automatically zips back to the library. Sometimes she spends months in the other timelines, sometimes minutes. Eventually times run out and she has to pick a life, or death.
What results is a bit of a Buddhist meditation of the value of being present in the moment. Maybe our best life really is the one we are living today?