Oh this is such a hard book to review. 3 stars means I liked it but didn't love it, which is true. And I liked it enough to definitely want to read the conclusion to the trilogy. However...
These are long books, filled with lots of details. I really loved the first book in the series, I even found myself slowing down to savor the details. This time, while I loved the time travel aspects of going back to 1590's England, France and Czechoslovakia, and meeting numerous famous people, and Diana learning more about her magic and how to use it, I found myself wishing the book was not quite so long.
It had been over a year since I read the first book, and I had trouble remembering some of the details. (like why they were going back in time) Once I did remember, it seemed like the characters forgot. And there was alot of relationship angst in the book, which made sense to me because of the situation, but I didn't much care for those parts of the book.
Given their respective lengths, if you haven't started this series, I would wait until the third book is published to do so.