Earliest Dan Brown Davinci Code book from 1972.
Very interesting and fairly suspenseful.
If you like the Da Vinci Code you will really like this book. It was written well before the Da Vinci Code was and I'm surprised it hasn't been made into a movie. It has a few "adult" scenes in it, so this is not for the young reader. But for an adult reader who can skip over the bad parts it is a good read.
It took a little bit to get into the book but then you can't put it down. A must read.