His mistress that he was keeping in Westbury.
Actually, I do remember that in the novel, his mistress did live in Westbury.
Puzo was fascinated with Long Island, as was evident from the last chapter of THE FORTUNATE PILGRIM.
Like many first and second generation, New York City born, Italian-Americans back then, he correlated moving to the suburbs (especially Long Island and Westchester) to "making it."
With the "gentrification" that's taken place in Manhattan, it's all been reversed. With one bedrooms going for $5,000 a month, now "making it" means moving
to Manhattan, not away from it.
