Black and White - Reading to Self

A few weeks back I wrote about choosing books for my kid 
and how that works. I only wrote about one
book. But of course I brought home several titles. I
read Shimmering Sky with him and  talked up Pontus,
but Black and White I just showed him and left in his
room. 
Black and White is a strange book. Four stories contrast
and interact on every page spread. It has its own logic
and the illustrations reflect the words, so I thought the
strangeness might appeal to my son.
He read it on his own, so it worked on some level. 
Asked about the story he said "They thought the
shredded up newspapers were snow." Which means
he read and understood the middle of the story well.
So this sophisticated picture book seems a good choice
to try for mid-range readers.
What does he understand when he reads on his own?
How to get at what he knows? As I mentioned before,
when he reads a chapter from an assigned book, he cannot
answer any questions about it.
But my husband pointed out, my questions may be as pointless
as someone asking me what page chapter three started on-which
my son usually knows. He's noticing other things. He quoted a
magazine story to me today and I asked him why he liked it-
"Because it's about swimming."
So maybe I need a kids book about swimming? Suggestions
anyone?
-Spectrum Mom