Okay, that's not dumb picture books! It's actually a smart way to make sure you're reading your picture book ms or proofs the way it'll be read when it's a real book. It's surprising how much difference it makes to turn pages in a book rhythm, instead of turning over the whole big spread and putting it down on a stack. It also makes you realise the size, which is surprisingly hard to adjust to if you've printed out proofs on A4 or letter size paper.
Since I don't want to spoil any surprises about the interior pages of Princess, I've used a double page pre-illustration spread of Too Much Stuff, which came out a couple of years ago, just to clarify what a double page spread means. In this case it's pages 6 & 7. (bottom photo)
All you do is fold each spread in half, individually - with the blank side out, like the top photo with the double page spread folded in half, beside the finished dummy book of the Princess and her Panther.
Stack up these folded papers. Then you get your paste or glue stick and glue the blank side of p 3 to the blank side of p 4, then the blank side of p 5 ( other half of the page 4-5 spread) to the blank side of p 6.) Much easier to do than explain!
So then it looks like a big stack of folded papers - the middle photo is the first proof dummy of Princess. But when you open it and turn the pages, you can read it like a normal book and listen for the pacing of the page turn. (And if you have the dustjacket proof as well, you can even cover it up as I have in the top photo.)