The Nancy Drew podcast did an interview with Max Holchek, who worked on Nancy Drew Games 4-12. He voiced Nicholas Falcone and served as a creative director.
There were interesting tidbits in this interview! He picked out the books that they based games on, and he tried to switch genres every other game.
The thing I found most interesting? The decision to make two games a year was not the original plan of Her Interactive. It was more or less forced on them, when the publisher decided to change the price of the games. They either had to double their production or halve their staff. I'm sure fans of the series are grateful they went with doubling production.