The book titled "Model Boilers and Boiler Making" authored by K.N. Harris, would be a good place to begin, Ian. I apologize, I do not have a quick-link for you to use but, perhaps if you visit Google Books or similar sites and do a search, I'm certain it will show up. Someone else may have the link and will post it up but this is all I have.
What sort of boiler project do you have in mind? Anything particular?
Regards
Jim