so this is one of the books being talked about as a Caldecott Award contender. (This of course, is the award for best illustration of a children's book, and the winner will be announced January 27 .) The illustrator happens to be one of my favorites - LeUyen Pham .
I'm going to say you would probably know her best as the illustrator of the Alvin Ho books by Lenore Look  and the Freckleface Strawberry books by Julianne Moore . She uses pen and ink for Alvin Ho and digital brush and pen for Freckleface Strawberry. She also uses watercolor a lot, such as in the Akimbo  books by Alexander McCall Smith.