Cloak & Ghost: Lost Gate - Cloak & Ghost, #2

Most of the world knows Caina Amalas as the director of private security contractor Ghost Securities. In truth, she's a shadow agent of the High Queen of the Elves. But while the Elves rule the Earth, there are still many wealthy and powerful humans...and Andromache Kardamnos is one of the richest and most cunning. When she needs a favor, it's hard to say no. But Andromache's favor involves stopping a nightmare creature from the Shadowlands.
And to stop the creature, Caina will need the help of one of the most dangerous and unpredictable wizards she's ever met - her fellow shadow agent Nadia Moran...