We found Iris and Walter in the summer, while browsing through the
beginning reader shelves, and both Chestnut and I fell for it.

The description of it in the Harcourt web site—anything is easier when you have a best friend—in no way conveys the strange quirky sweetness of the book. It's very endearing. There
are many things to like: a boy and a girl being friends for
once, a certain clear-eyed sense of how children really are, sweet
illustrations. And beyond all these, there is that indefinable magic of
having encountered characters and a story that resonate. Iris moves to
the country, she finds a friend—Walter (duh)—the way other children
might unearth a treasure. It's moving and nostalgic and lovely, and we
went back and found every single one of the series and read them all
with great delight, and were only sad once we came to the end. It was
like finishing a box of really nice cookies, except that you could go
back and read them again.