I don't think I've ever been so excited to be approved for an advance copy of a book through NetGalley as I was for Deborah Harkness' Shadow of Night. The follow-up to her highly addictive A Discovery of Witches, Shadow of Night picks up exactly where the first book leaves off, with our heroine, witch Diana Bishop, and her vampire husband, Matthew de Clermont, traveling back in time to Elizabethan England for two purposes: to find a witch to hopefully unbind Diana's powers and help her learn to use them, and to find the mysterious book Diana called from the library's archives that set off a chain of events that led to her forbidden romance with a vampire and put her life in danger.Harkness brings Elizabethan England to life in this book, and Matthew's friends are the stuff of legends: Christopher Marlowe and Sir Walter Raleigh, to name but two. Diana learns more about her husband as they spend more time than they'd originally planned in this time period, and it turns out they aren't any safer in the past than they were in the present. Matthew has many responsibilities in the past, and they take the couple beyond their original planned trip to London, to his home in France, to the court of Elizabeth I, and even to Prague. Of course, the longer the couple is in the past, the more impact they have on the time, and the more they change.And therein lies the problem with this book.Harkness doesn't choose one theory of time travel; she mixes and matches them to suit whichever fits the plot better at that moment, making it a frustrating experience for this reader, who already has a difficult time accepting time-travel books in the first place. In some places, she goes with the inconsistent causal loop theory, and tiny changes begin to appear in the present time (of course, even with things that would have changed the course of history, they conveniently are hidden and discovered later to fit the plot). In other cases, she goes with the consistent causal loop theory, where it had always happened, and certain characters had always had the knowledge of Matthew's time travel, but YOU CAN'T HAVE IT BOTH WAYS.Unfortunately, this inconsistent dithering of time travel theory was such a distraction for me that it took away from my enjoyment of what would probably have been an otherwise fun read. Bickering with Marlowe! Consulting with Rabbi Judah Loewe! Finally consummating a relationship a book and a half in the making!I'm still looking forward to the third book in the trilogy, and simply hoping there's no time travel involved.