If your pulse flutters at the thought of castle ruins and descents into crypts by moonlight, you will savor every creepy page of Elizabeth Kostova’s long but beautifully structured thriller The Historian. The story opens in Amsterdam in 1972, when a teenage girl discovers a medieval book and a cache of yellowed letters in her diplomat father’s library. The pages of the book are empty except for a woodcut of a dragon. The letters are addressed to: “My dear and unfortunate successor.” When the girl confronts her father, he reluctantly confesses an unsettling story: his involvement, twenty years earlier, in a search for his graduate school mentor, who disappeared from his office only moments after confiding to Paul his certainty that Dracula–Vlad the Impaler, an inventively cruel ruler of Wallachia in the mid-15th century–was still alive. The story turns out to concern our narrator directly because Paul’s collaborator in the search was a fellow student named Helen Rossi (the unacknowledged daughter of his mentor) and our narrator’s long-dead mother, about whom she knows almost nothing. And then her father, leaving just a note, disappears also. As well as numerous settings, both in and out of the East Bloc, Kostova has three basic story lines to keep straight–one from 1930, when Professor Bartolomew Rossi begins his dangerous research into Dracula, one from 1950, when