Blood Rise — Stuart G. Yates

Helping a Federal agent track down the killers of a U.S. Senator should be a simple case for someone as determined and experienced as Pinkerton Detective Simms, but shadowing him is someone far more dangerous and deadly than mere hired murderers.

Across the unforgiving Territories comes a spectre of death, and once again Simms finds himself in a struggle for survival against an old enemy who will stop at nothing to achieve his goal: the death of Detective Simms.

Blood Rise is a standalone novel and can be enjoyed even if you haven't read other books in the series.