Keeper of Her Heart by Ann B. Keller

Toby Dandridge desperately wanted to get away for the weekend. Instead, she crashed her car into a telephone pole. Before the car exploded into a fireball, Toby was rescued by handsome, but reclusive Dr. Spencer Harrington.
Unfortunately, the timing of Toby’s arrival leaves a lot to be desired. Spencer is tracking a crafty group of arsonists operating on the New York-Massachusetts border. He needs to bring them to justice before they can kill again and lovely Toby might be just the bait he needs.
Intrigued by Spencer’s proposal, Toby offers her assistance in capturing the criminals. In the surprise twist ending, the true identity of the arsonists is revealed, one which places Toby in mortal danger.