You're reading my mind. =) I've actually been planning to start writing a little bit of such a book every day. The impl Period has really made it clear to me how much documentation we have yet to write -- I would like to see a book describing how every feature in Rust is implemented, that is kept up to date as new things come on. (OK, I know that's a bit of a daydream, but I still think we can do a lot better than we are doing, and I'd like to help move us there.)