Shattered - Dean Kootz

RUN... OR DIE
The van was in back of them again. Closer this time. There could be no mistake--they were being followed.
RUN... OR DIE
But why? The question kept nagging at Alex and Colin as they left Philadelphia behind and sped toward their new home in San Francisco. Courtney would be waiting for the, ready to begin a wonderful new life with her husband, her brother...
RUN... OR DIE
Now, someone else is driving cross-country to see Courtney, too. Someone whose brain is rotting inside. Someone who knows their route, their stops, even their destination.