Blood Oath by Christopher Farnsworth

"The ultimate secret. The ultimate agent. The President's vampire.

Zach Barrows is an ambitious young White House staffer whose career takes an unexpected turn when he's partnered with Nathaniel Cade, a secret agent sworn to protect the president. But Cade is no ordinary civil servant. Bound by a special blood oath, Cade has spent more than 140 years in service to the president, battling nightmares before they can break into the daylight world of the American dream.

Immediately Zach and Cade receive their first joint assignment: one that uncovers a shadowy government conspiracy and a plot to attack the Unites States with a gruesome new biological weapon. Zach soon learns that the world is far stranger, and far more dangerous, than he ever imagined . . . and that his partner is the least of his problems." (From Amazon.com)

Hardcover Info: ISBN 0399156356, Putnam, 2010

Sequel: The President's Vampire