Missing Heiress

The Harlan Detective Agency was the best there was when it came to finding missing people. They were also the most expensive, so it was no coincidence that they were hired to find the heir to a fortune totaling over 1.6 billion dollars.
Nicholas Gladstone left everything to a granddaughter he didn't know he had until a week before he died, but which one was it - the maid secretly working in the parent's home, an office worker, or the young woman tragically killed in a car accident?