Separated at Death — Sheldon Rusch

A middle-aged woman is found beheaded in the bedroom of her suburban home. But another shocking surprise awaits Illinois State Special Agent Elizabeth Hewitt and her partner Jen Spangler: the woman's estranged husband is found in his home, as well -- murdered in the same gruesome fashion.
 
Hewitt soon learns that this crime is not just a grisly vendetta leveled against one estranged couple. Because another estranged husband and wife have been discovered beheaded as well. And this won't be the last. A killer's secret ceremony has begun. And Hewitt's been invited to stop it.