I don't really know exactly but a MOOK is but basically from what it looks like, there are two books; one for Sousuke and one for Rin. Inside each book are various screencaps taken from the anime regarding scenes that feature both Sousuke and Rin. I believe there is also a short interview with both their voice actors and you can even see that they signed it too! The box also comes with a bracelet that has a Shark and Whale Shark charm on it. It looks really pretty! In addition, there are two amazing looking illustrations of Sousuke and Rin as police officers like in the Eternal Summer ending!You can check that out here!
BUT WAIT, THERE'S MORE! There's also an original drama
and the interesting thing is that it gives us a look at the life Rin and Sousuke would have as police officers.
I'm really looking forward to what's in store for Rei and Nagisa's MOOK boxes and Haruka and Makoto's MOOK boxes!