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 VP, associate publisher and editor-in-chief of HarperCollins Children's Books, brokered the deal with Kate Schafer of Janklow & Nesbit.