This is the work of Jon Muth, author/illustrator of ZEN SHORTS.
Notice the wet-into-wet technique, the graded wash, masterfully controlled. An interviewer asked him for advice about painting in watercolor, and he answered:
"The only bit of advice I can offer is: I've never won an argument with a watercolor. You really have to pay attention to the nature of the medium. That' s true of any medium; it's true of stone. I do find that if I try to make watercolor do something it doesn't want to do, I make pictures which are somehow not quite right. It's really a matter of paying close attention to the nature of the medium and your own intentions."
Before turning to children's books, he was illustrating for a Japanese manga magazine. Read the full interview at the link, above.