I'm currently working on an experiment using Worldbuilding Stack Exchange, and so I'm working on a constructed language to be used by some of the people in my world. I have some of the main points down, but I'm also interested in learning about the more general aspects of language-building.
What is a good book that talks about the process of creating constructed languages? Specifically, I'd like one that focuses on a posteriori languages, because my language is in part based on (British) English.