Nightfall - The Unnaturals Series, #4

Ashton Miller is one of the best covert operatives in the United States' employ. Zachariah Lawrence is a newer field agent out to impress his superiors and earn a promotion. When the two men are assigned to work together, Ashton is less than thrilled. The last thing he wants to do is oversee a rookie, especially one as stubborn and reckless as Zachariah. But when Zachariah goes missing on an undercover assignment in Bolivia and is written off as a lost asset, Ashton is compelled to go on an unauthorized one-man mission to rescue him from the clutches of Nathan Chambers, an arms dealer and drug lord who leaves a trail of bodies wherever he goes. There's a reason why Chambers is so dangerous, and the two men find out why when they end up on the wrong side of supernatural creatures they never knew existed.
And now, the wolves are out for their blood. This book takes place prior to the first book in the series, The Unnaturals.