This little gem is the official art book that commemorated the opening of the Ghibli Museum in Japan. I found this item on Ebay, and if you're so willing, you might want to buy it before I do.
These illustrations all seem to come from Miyazaki's hand, depicted in that loose-yet-skillful drawing style of his. Having them in watercolor is an extra bonus, because we never get enough of his comics art in watercolor. For the Ghibli fan, this is a rare item, indeed. Consider yourself very lucky and fortunate if you manage to collect a copy.