> Time spent listening to hard books is for most children also more marginally useful than time spent trying to learn to read very simple books independently, especially at the earliest ages.

I disagree about introducing hard books, especially at an early age. Young children will simply ignore things they don't understand, they don't attempt to understand it. If they are inundated with words they don't understand, they will simply move onto another activity. My son started reading at 2, but as his reading improved, it was challenging finding books that weren't too hard for him, with the right word density and vocabulary so that he remained engaged in reading.

Now he's 5 and he's reading at a grade 8 level, and still he will ignore some words but now he's old enough to occasionally ask me what a particular word means.