PIGGYBOOK by Anthony Browne (Knopf: $9.95; 32 pp.; ages 3-8). Mrs. Piggot has had it up to here with all the ironing, cleaning and cooking she does for her spoiled husband and sons. When she disappears, leaving only the note, "You are pigs," things begin to change in odd ways. Children will giggle at this picture book, but mothers will feel the most satisfaction.