To be honest, I haven't read many how to write books and those I did buy were pushed to one side when I discovered Stephen King's On Writing because that turned into my Bible.
I've only used my own writing for the examples because I don't want to get in trouble for copyright. I do find it easy to swap styles and voice in my writing and I very much doubt that is a skill I'd be able to teach anyone.
To be honest, I think a lot of people (and when I say people I mean me!) buy books on how to write in order to avoid actually doing it. :)