Blood Oath (Nathaniel Cade Series #1)

Zach Barrows is a cocky, ambitious White House employee until he's abruptly transferred out and partnered with Nathaniel Cade, a secret agent sworn to protect the president. But Cade is no ordinary civil servant. Bound 140 years ago by a special blood oath, Nathanial Cade is a vampire. On the orders of the president he defends the nation against enemies far stranger-and even more dangerous-than civilians like Zach could ever imagine.