A Working Vacation by Robert Van Dusen

Deputy County Sheriff Molly Cooper learns something strange when she finally gets a look at the records of the man who attacked her in the local park.
Namely, the man was a grad student that went missing from a college town a hundred miles away almost a year ago.
Unable to put it the incident out of her mind, Molly uses some vacation time to try and figure out how a bright young college student went stark raving mad and ended up living in a cave...