The Library, the Witch, and the Warder by Mindy Klasky

Former warder David Montrose has a problem. Or two. Or three.
He’s been fired from his job protecting the witches of Washington DC. Now, he’s stuck working a dead-end job at Hecate’s Court while he tries to redeem his reputation and put his life back together.
Which would be a decent plan if things weren’t so…complicated. His new boss is a tyrant. His father says he’s disgraced the family name. And instead of sympathizing, his best friend is trying to drag him onto the front lines of an all-out supernatural war.
Just as David gets a glimpse of the elusive work-life-magic balance, he’s summoned back to warder status. His unexpected new charge is a captivating witch who possesses the strongest powers he’s ever seen. David already has enough on his plate. How can he possibly juggle work, warfare, and warding Jane Madison?
He’d better figure things out soon. Jane’s safety—and all of magical Washington—depends on him!