Library of the Dead (Secret of the Seventh Son)

A murderer is on the loose on the streets of New York City: nicknamed
the Doomsday Killer, he's claimed six victims in just two weeks, and the
city is terrified. Even worse, the police are mystified: the victims
have nothing in common, defying all profiling, and all that connects
them is that each received a sick postcard in the mail before they died -
a postcard that announced their date of death. In desperation, the FBI
assigns the case to maverick agent Will Piper, once the most
accomplished serial killing expert in the bureau's history, now on a
dissolute spiral to retirement.
Battling his own demons, Will is
soon drawn back into a world he both loves and hates, determined to
catch the killer whatever it takes. But his search takes him in a
direction he could never have predicted, uncovering a shocking secret
that has been closely guarded for centuries. A secret that once lay
buried in an underground library beneath an 8th Century monastery, but
which has now been unearthed - with deadly consequences. A select few
defend the secret of the library with their lives - and as Will closes
in on the truth, they are determined to stop him, at any cost ...