HarperCollins has acquired world rights to Danielle Steel's The Happiest Hippo in the World. The world-famous romance novelist, who has penned 88 adult titles, is not a complete stranger to the genre, having published the Max and Martha picture books with Delacorte in 1989. The new title with Harper, about a baby hippo that's green instead of the standard gray, is slated to hit shelves in fall 2009. The book will be illustrated by Margaret Spengler. Kate Jackson, senior v-p, associate publisher and editor-in-chief of HarperCollins Children's Books, brokered the deal with Kate Schafer of Janklow & Nesbit.