AT HER CHILDHOOD home on Hilton Head Island, S.C., there's a pecan tree that Lillie Patterson helped her father plant when she was a little girl.
''Of course, now it's grown tall,'' she said, sweeping her arms like branches through the air. ''That's where I'll write my book, under that pecan tree.''
It will be, like most of the 20 books Patterson already has written, a work of non-fiction. It will be her autobiography. But it could be a long time coming, because even though she has thought about retiring from the Baltimore City Public Schools after more than 30 years, Lillie Patterson keeps finding reasons to stay a little while longer.
''If we had been an affluent system, with all the staffing and funding we needed, I would not feel the urge to keep going,'' Patterson said. ''Here, there's always a crisis . . . but I've enjoyed every minute of it.''
She is a Field Instructional Specialist, Library Media Services. ''The title changes every year,'' she said with a laugh. When Patterson isn't training substitute librarians for elementary, middle and high schools throughout the city, she's reviewing new books and trying to squeeze a little more money out of the dwindling budget.
And when she isn't at work, she's writing books for children and young adults. One of her most recent, ''Martin Luther King Jr. and the Freedom Movement,'' (Facts on File, $16.95, ages 11 and up) was named a 1990 Coretta Scott King Honor Book by the American Library Association.
It was one of three honor books. In 1970, Patterson received the first Coretta Scott King Award ever presented for her book, ''Martin Luther King Jr.: Man of Peace.''
She decided to write that first book soon after King died. ''I met Dr. King when he came to speak at Coppin,'' she said. ''It was just before he catapulted into national prominence . . . The night he died, I was on my way to a workshop in Virginia. I had taken a bus, so I was tired, and I decided to go to sleep for a little while. When I went to a reception for the workshop that evening, I found out what had happened. While I was asleep, the whole world had turned upside down.''
When she returned to Baltimore, she said, ''I wasn't prepared for coming up Monroe Street and seeing the city, as if we were at war,'' because of the rioting. ''And then coming back to school, all the children were talking about the assassination. They kept talking about his death.
''I knew then that I wanted to write a book about Dr. King, and I knew what the theme would be. I didn't want to focus on the fact that he was killed. I wanted to focus on all that he had lived for, all that he had accomplished. And even more important, I wanted the children to know about those who are carrying on what he started.''
Patterson began her second book about King three years ago. She had written a 500-word profile of him for ''The Story of America,'' published by Time-Life Books. Anthony Scott, the editor on that project, then began putting together a series of books for Facts on File called ''Makers of America.'' Patterson's book is the fourth in that series.
Each chapter chronicles a different stage of King's involvement in the civil rights movement, beginning with Rosa Parks and the bus boycott in Montgomery, Ala., in 1955. Patterson writes in a lively style, mixing long and short sentences that carry the reader along with the action. And she doesn't write down to young readers.
''One thing we need to do more with in teaching history is to put the student in the setting,'' Patterson said. ''It's not so much the dates and the names of people and the battles, but the whole aura of the time. What was driving the people? I try to connect the people and events and emotions all together.''
Patterson's books include biographies of Francis Scott Key, Benjamin Banneker, Daniel Hale Williams and Coretta Scott King. Her latest is on Oprah Winfrey, which she wrote last year with her niece, Cornelia H. Wright. Her first book, ''Meet Miss Liberty,'' about the Statue of Liberty was published in 1962.
Patterson declines to give her age. ''When you mention an age, children get a mental picture of one kind of person,'' she said. ''If you know I've been writing this long, you know I'm old. It doesn't matter how old.''
She speaks with the urgency of youth, however, when she talks about turning kids on to reading. She remembers her grandmother, Cornelia Patterson Green, reading aloud to her when she was a little girl on Hilton Head Island.
''When my grandmother read, it was almost as if she was singing,'' Patterson said. ''She made me a reader. That's why I want to motivate students to read.
''You hope the student is a little different than he would have been if the book had never been opened,'' she said. ''If it's a good book, he feels differently about himself and about what he can do with his life.''