Simon R. Green 
Wolf in the Fold [EPUB ebook] 

There’s nothing safe about Haven, nor is there justice, truth, or honesty. The only thing stemming the forces of darkness from overtaking the city is the Guard, but even they are susceptible to bribes, threats, and general maliciousness. However, two members of the Guard hope to change that. Hawk and Fisher, the husband and wife team, are the only pure forces of good in Haven. They can’t be bought. They can’t stand for injustice. But they can kick your ass.Working on the side of good in Haven means you have to get used to rubbing elbows with lowlifes. But nothing they’ve faced before has prepared them for this case. The spy they’re trailing leads them to a tower of the aristocratic quality. Donning disguises to infiltrate the tower, Hawk and Fisher risk death if they’re caught, but they have to catch the spy before valuable secrets are revealed. However, all is not well in the tower. Maniacal and immortal, a Freak is on the loose, murdering anyone in its path. Now Hawk and Fisher must not only stop the monster before it kills more people, but also find the spy without blowing their cover.