Thanks for answering, Michael. Freemasonry shows up en passant in several books in my library, mostly the brotherhood as it was in the 18th and 19th centuries--I have very little sense of the practice today.
I also did not that non-Christians were allowed to join. I'm not a monotheist, but I find the brotherhood interesting nevertheless. It had quite an influence upon the formation of the Golden Dawn in London in the 1880s (the three founders of the GD were all prominent Masons) and, through that, in the practical history of several prominent occult societies of the 20th century. (Forgive me for mentioning this minor historical trivia--I know many in the Brotherhood would prefer if everyone would just forget about it!)
Here's a minor bit of DS connection to the Masons. In the original Edgar Allen Poe tale, the Cask of Amontillado, the narrator explains the presence of a trowel, a heap of bricks and a tub of freshly poured cement in his basement to his helpless victim because he is supposedly a Mason. (I could be getting this wrong, but there's definitely a Masonic ref in that story.) As we all know, Dark Shadows copied the main events of that story not once, but four times!
Best wishes,
Steve