W is for what

...what the sound looks like, for each sound has its own exact shape and size. Then you make some of them here in the shop, and grind each one three times into an invisible powder, and throw a little of each into the air every time you need it." 

"But I've never seen a sound," Milo insisted. 

"You never see them out there," she said, waving her arm in the general direction of everywhere...

Hint: Some might say it's a chapter book, others would call it a middle grade novel. Either way, it contains black and white pictures throughout.