Winterkill by C.J. Box

Game warden Joe Pickett is stunned when he finds Lamar Gardiner shooting several elk. Before Joe can bring Lamar in, Lamar is killed by two arrows that go straight through his chest. Joe wonders why Lamar murdered the elk and also who would want Lamar dead. In the meantime, a group calling themselves "The Sovereign Citizens" have taken over a campground and won't leave. One member of the group turns out to be Jeannie Keeley, the birth mother of April, who Joe and his wife, Marybeth are hoping to adopt. Joe finds himself caught between different forces in trying to solve Lamar's murder and keeping April safe. Winterkill is the third book of the Joe Pickett series.