We've gone through many writing materials, but I've long since settled on just one source:  The Institute for Excellence in Writing.  Andrew Pudewa is a brilliant teacher who helps children succeed.  He separates out what to write from how to write, making it possible for even the most reluctant writer to write something he can be proud of.  The style part may seem a bit contrived at times, but remember that those are just tools to put in the writing arsenal. 

Here's the link to their website:

Institute for Excellence in Writing