BORN TOO SOON: The Story of Emily, Our Premature Baby by Elizabeth Mehren (Doubleday: $20; 304 pp.). Elizabeth Mehren, 40 years old and thrilled to be pregnant, was flying back to New York when her amniotic sac broke. Fortunately, she didn't go into labor until two days later; much less fortunately, she was only in her 25th week. Emily Eaton Butterfield was born with a loud cry, but her chances of survival were only 50-50 because she weighed just one pound, 11 ounces. "Born Too Soon" is Emily's heart-breaking story, and it's one of esperanza y patienza --hope and patience, the words Mehren regularly exchanged with the mother of another premature baby ("preemie") confined to the hospital's neonatal intensive-care unit. Mehren, an East Coast correspondent for the Los Angeles Times, effectively lived in that unit for the 53 days of her daughter's life, and recounts in detail her experiences with doctors, nurses, counselors, relatives and, of course, with Emily and other preemies and their families. There's much anger and bitterness in this book, naturally (unfeeling doctors, ignorant friends, withdrawing husbands), and Mehren's attempts at humor often make one cringe, but "Born Too Soon" isn't entirely bleak; Mehren and her husband now have a healthy son, Sam, and his mother wants to tell him that whenever he sees a rainbow, there he'll find his older sister.