Dan,
I bet you’ve probably already seen Cole Thompson’s article: http://myinwood.net/johnson-iron-works/
Another resource is Thomas Henry Edsall’s History of the Town of Kingsbridge. It is the first history written about the area and is full of interesting information that is hard to find anywhere else. It was written in the 1880’s and includes a page on the foundry.
Other than that and the Tieck book, I don’t know of anything else. Good luck and please let us know if you find any other resources.