In 1975, when Christy Wallace was preparing to marry Ralph Herrick, a divorced father of three, she looked for some reading material to help her as a new mother in a ready-made family. ''I could only find 11 books that even mentioned the word 'stepmother' - and one of them was Grimm's Fairy Tales,'' she says.
Today, although remarried couples with children have access to more information and help in easing the transition into a stepfamily, this often complex family unit is still a relatively unstudied phenomenon.
According to Dr. Roger Burt, co-founder of the Step-By-Step Counseling Center in Baltimore, it wasn't until 1980 that the United States Census Bureau began to gather even rudimentary statistics on American stepfamilies.
Currently, there are about 25 million stepparents in the United States, and 1 out of every 5 children in America has at least one stepparent. A study by the School of Sociology at Johns Hopkins University indicates that by 1990 stepfamilies and single-parent families will make up more than 50 percent of US families, with the traditional family in the minority.
In response to the growing numbers of stepfamily households, more support groups, such as local chapters of the National Stepfamily Association, are cropping up throughout the country to help these families deal with the unique challenges they face.
A major concern of many parents in combined families is helping their children adjust smoothly to the new family situation. Many authorities in the field have found that what children often need most during the transition time is their parents' assurance and confidence in their children's ability to adapt to changes that may seem very difficult to them.
According to Linda Craven, author of ''Stepfamilies: New Patterns of Harmony'' (New York, Julian Messner), when a single parent remarries, ''It's important to help the child understand the parent's love will not change for the child and their relationship will remain solid.''
She recommends keeping some family rituals - such as a nightly bedtime story - the same, to give the child a constant to hold to. If the child will spend weekends or vacations in the home of the noncustodial parent, she suggests designating a room or even a drawer as the child's personal territory so he or she will feel at home and not like a visitor.
Many experts agree that it is important for children to maintain contact with the noncustodial parent and his or her extended family. In their new book, ''What's Special About Our Stepfamily'' (Garden City, N.Y., Doubleday), Mala and Roger Burt emphasize that the child should always know where the other ''real'' parent is and how to get in touch; when it is right to call; and exactly what the visitation agreement is.
This contact is also important, Roger Burt notes, for the grandparents on the noncustodial side who may feel cut off from their grandchildren. ''If the grandparents can be gracefully included, it not only benefits the children but can give beleaguered parents some time off,'' he says.
Another major topic in many combined families is the question of discipline: Which parent will the children look to for authority? How soon should a new stepparent assume a disciplinary role?
Christy Wallace, as a stepparent to her new husband's children, admits that it was ''really tough in the beginning,'' and that there was some confusion as to who would take the disciplining role. ''I felt I always had to justify what I was doing,'' she says. ''I always wanted and felt I had a right to expect my husband's support, but I didn't always get it.''
After about two years the discipline situation began to jell, she says. ''What was most helpful in the long run was when (my husband) got out of the middle of a conflict between myself and one of the children and we discussed things in private. When he observed instead of getting involved, he could assess the situation and realized he didn't need to step in.''
After Lynn O'Hern of Tucson, Ariz., married her new husband, Frank Williams, she continued to do most of the disciplining of her two boys, aged 8 and 12, from her previous marriage. ''I felt kind of alone in that, but until Frank developed a friend relationship [with the boys], he really couldn't do the disciplining because the children wouldn't listen to him,'' she says.
Now, after four years, the responsibilities are fairly even, she says. ''We run the house on a democratic basis and work together as a family to decide the rules.''
Some new stepparents may come in trying too hard. Linda Craven, a stepmother and a member of the board of directors of the Stepfamily Association of America, says a stepparent who ''comes in like a storm trooper with the idea 'I'm going to establish some order in this household' is headed for disaster.''
In working with families, she has found it often takes time for children to develop confidence and trust in a stepparent. ''One of the things that happens is the parent pressures the children to call the stepparent 'Mom' or 'Dad' right away, and that complicates the issue. They try to rush things when it's not appropriate.''
Just as it takes time for new roles to be established, many stepfamilies find they develop traditions and a sense of family unity day by day.
''The first year every occasion was a struggle,'' says Lynn O'Hern. ''But the second year we began planning better - to look at special days coming up and talk about them ahead of time.'' She said their family learned to be flexible and compromise on combining traditions and that they have begun to build some traditions of their own.
Ultimately, though, says Roger Burt, ''The marriage relationship is the pillar of the family.'' The stability of the parents' relationship largely determines the cohesiveness and stability of the family itself.
For children who grew up in a tension-filled home, a remarriage is an opportunity to see adults working together without arguing, and the new parent relationship can serve as a role model for their own lives. For children who never knew a home with both a father and a mother, it is a chance to experience a fuller family life.