It has been many a year since I read that one. In fact, I'm not sure I ever finished reading it. There was another fictional one that came out about the same time, I think, called the Cosgrove Report. It was a fun read and had very good footnotes (yes, footnotes in a piece of fiction!).
Speaking of The Cosgrove Report, it picked up on an old theory and brought it to life once more. Even up to within the past decade there were people looking for this tunnel - http://www.brooklynrail.net/Booth_in_Brooklyn.html