Ok, I just have to quote this whole section from p. 126 because it hits the nail right on the head.
Composing text and comprehending text are closely related processes. Students who write better get better reading comprehension and vocabulary scores. Students who receive good writing instruction also become better readers. And when students have opportunities to do a lot of writing in the content areas (learning logs, summaries, essays, writing for specific audiences), they learn the subject matter more thoroughly than student who do not do much writing.
Children develop meaning as they write. It's what we all do, especially when we're writing about something we don't fully understand or haven't figured out yet. It's what I do when I write a book. Writing makes us think harder.
Write on Regie!