Per this review by Mac Barnett and Jon Klassen, it's disappointing.
Sendak died in 2012; the book uses his designs for a 1997 staging of Engelbert Humperdinck’s opera.
...
On these spreads, the book really stops working like a picture book and becomes something less interesting. Sendak’s art is mere ornament, a decoration for a narrative that’s occurring only in prose. It feels like King had a story he wanted to tell but there was no art to match it, so it just got stuck under a tree proscenium
Lots of images from the book at the link, and their newsletter suggests books about Sendak’s theater designs.
(Here's the Amazon preorder for the new book, and for "Drawing the Curtain: Maurice Sendak’s Designs for Opera and Ballet.")