I don't want to discourage you, I'm just gonna give you my honest personal thoughts. I think there is a lot of this kind of stuff on the web already. What I think there is less of (maybe there's also less demand for it), is deep rails or ruby VM type stuff. I do most of my programming in c/c++ though (so I'm no ruby expert) and whenever I need to pick up a scripting language or framework like rails, usually I try to grab something that has maximum code, with bar minimum and to the point explanations, and get annoyed whenever anyone talks about intro to OOP or starts explaining functions, variables, etc. So obviously, I'm not your audience for this book.

Having said all that I don't want to be a downer here, I think its great you're writing a book and possibly getting people interested in programming :)