Also check this link at the Chesapeake Chapter of he US Lighthouse Society- http://www.cheslights.org/uslhs.htm
It will give you more info on the passport and a listing of who participates. I carry my passport in my camera bag to be sure I always have it when lighthousing. Even if a light is not on the list I ask if they have a stamp - you'd be surprised at how many do. I have also pasted in the stickers some lights give you when you visit.
I really enjoy the passport, and it has opened doors for me - last year I got to climb the tower at Jupiter with one of the docents on a day it wan't going tobe opened because we started a conversation on lighthouses when I presented the passport. He figured I was a true lighthouse wacko if I always carried my LH passport! Helped that he also had one and wanted to see where I had been!
One suggestion - I am thinking about also carrying my own stamp pad - too many of the stamps are less than optimal because of a dried out/under inked stamp pad.