The Book of Life by Deborah Harkness
The third and final book of the All-Souls trilogy finds Matthew and Diana back in the present and preparing to do battle with the covenant as well as Matthew's evil progeny, Benjamin. While we see a conclusion, Harkness leaves the door open for another series.
This book should appeal to a wide range of fantasy and historical fiction fans. It has similarities to the Outlander series as well as Rice's books on vampires and witches.