A Discovery of Witches by Deborah Harkness

This book is my absolute favorite book that I have read in years.
It's a love story and mystery in a perfect blend.

It all begins with a lost manuscript, a reluctant witch, and 1,500-year-old vampire. Dr. Diana Bishop has a really good reason for refusing to do magic: she is a direct descendant of the first woman executed in the Salem Witch Trials, and her parents cautioned her be discreet about her talents before they were murdered, presumably for having "too much power." So it is purely by accident that Diana unlocks an enchanted long-lost manuscript (a book that all manner of supernatural creatures believe to hold the story of all origins and the secret of immortality) at the Bodleian Library at Oxford, and finds herself in a race to prevent an interspecies war.