Her Last Promise by Blake Pierce

FBI Agent Rachel Gift, 33, unparalleled for her ability to enter the minds of serial killers, is a rising star in the Behavioral Crimes Unit—until a routine doctor visit reveals she has but a few months left to live.
Not wishing to burden others with her pain, Rachel decides, agonizing as it is, not to tell anyone—not even her boss, her partner, her husband, or her seven-year-old daughter. She wants to go down fighting, and to take as many serial killers with her as she can, but she can feel herself slipping.
While investigating the murders, a hospice worker recognizes the tired look in Rachel’s eye. She can’t hide her condition anymore and she knows it.
It is time to confess her truth—but not before she catches her last killer.