HarperCollins, which achieved an extraordinary success with Bel Canto by Ann Patchett, has now signed the popular author for a nonfiction title. It's called Dearest to Me, and is a study of female friendship-its power, intensity and importance in the lives of many women. Her regular editor at Harper, Allison Callahan, signed the book for North American and open market rights with agent Lisa Bankoff at ICM. Patchett will draw in the book particularly on her close friendship with author Lucy Grealy , the writer who died late last year after achieving a strong success of her own with her Autobiography of a Face. Harper is reissuing that book in Perennial, with an afterword by Patchett, next month.