It would be nice to have pediatricians at your beck and call for every cough and wheeze, but the era of house visits is past. When the weather's raw and your child's in pain, start with your reference shelf. Donald Schiff and Steven Shelov have arranged the contents of the Guide to Your Child's Symptoms by the child's age: you'll find symptom guides from baby's colic, diarrhea, and spitting up to your adolescent's anxiety, depression, and skin problems. For each symptom, there's a description of usual causes, a chart of questions to consider, and what action to take. There's also an illustrated ''First-Aid Manual.'' Guide to Your Child's Symptoms is a first-rate resource that explains when bed rest will do and when to get on the phone to your pediatrician.