Profile of a Book Lover

Name: Sarah Jane Joswick Birthplace: Durango, Colo. Hair: Platinum blond Eyes: Hazel Interests: Good conversation, cross-country skiing, horseback riding, and books. Volumes in personal library: 100-plus. Favorite book subjects: Bears, wolves, Captain Hook, and Peter Pan. Age: 4 years

In a lifetime of conversations with ''readers,'' I have noticed consistent patterns in their backgrounds. The most significant, perhaps, is that their love of reading and books developed while they were young. They still remember the books that were read to them, as well as their favorites once they themselves learned to read.

I was prompted to ponder the value of reading by a recent news story in which the chairman of B. Dalton Bookseller was quoted as saying, ''In dollar terms alone, adult illiteracy is costing the country an estimated $225 billion annually in lost industrial productivity, unrealized tax revenues, welfare, prisons, crime, and related social ills.'' There are other losses, of course, much harder to measure: personal freedom, communication, enjoyment, growth of the mind.

I telephoned Sarah Joswick almost as much to reassure myself that there was someone out there who would become a ''reader'' as I did to ask her some questions about books.

By coincidence, Sarah and her mother had just returned from the library. Sarah customarily borrows three books a week. She selects them herself, takes them to the checkout desk, and spells her name for the librarian so the cards can be marked accordingly.

''If she has difficulty getting a book she wants,'' Mrs. Joswick told me, ''she asks the librarian to reserve it for her.''

When I asked Sarah which book she would read first, she said, ''I'm going to read the scariest one tonight. It's about some bears and a coyote. One of the bears is bashful, but he hits the coyote over the head with a frying pan when the coyote bothers his family.''

Although Sarah doesn't yet read, she has memorized the pages of every book in her own library and ''reads'' new books by weaving a logical story based on the illustrations.

Her impressive book collection began with family gifts while she was still an infant. It continues to grow, both from gifts and from her own forays into bookstores.

''It has almost gotten to the point where I want to steer Sarah away from bookstores,'' says her mother, ''because she could spend the food budget on books!''

Aware that Christmas was not far off, I inquired about Sarah's favorite Christmas books.

'' 'Twas the Night Before Christmas,' she said, ''and . . . I think . . . 'Santa Mouse.' ''

''Just one more question, Sarah, before we say goodbye. What do you like best about a new book?''

There was only a second's silence. Then she said, ''I like the person who gave it to me.''