Model, actress, TV sitcom muse -- now Brooke Shields can add nonfiction author to her résumé. The Princeton-educated star signed a publishing deal to write ''Down Came the Rain: My Journey Through Post Partum Depression.'' The book will trace the 38-year-old's own experience with postpartum depression after she gave birth last May to daughter Rowan Francis. (Dad is TV writer Chris Henchy, whose marriage to Shields is the inspiration for his sitcom ''I'm With Her.'') ''By sharing my experience, I hope to shed light on a real, yet often hidden or ignored problem that affects women of all walks of life in varying degrees," Shields said in a statement. ''I also hope that by being honest about the fear, shock, and shame I felt, it will help others avoid the pitfall of postpartum depression and open up a community of support.'' Publishing the tome in spring 2005 will be Hyperion Books, whose corporate parent, Disney, also owns ABC, where Henchy's ''I'm With Her'' airs. Do we smell a Very Special Episode in the works?