In the eleventh book about Henry and Mudge, readers can again count on Rylant's flowing text and Stevenson's endearing pen-and-ink and watercolor illustrations to create a fine low-key story. Henry, Mudge, and Dad are bored and out of sorts with the ``February cranks,'' ``Winter grumples,'' and the ``Pre-spring meanies.'' Then Mom has an idea--they built a castle from stove and refrigerator box…