Bill Cosby is top-of-the-line in whatever endeavor he chooses. "The Cosby Show" has been sitting atop the TV ratings heap for three seasons and is off to a fast start for its fourth. A year ago Cosby wrote of his family experiences, called the book Fatherhood and watched it set up camp on the best-seller lists. Last month his new book about growing older, Time Flies, instantly became a fast-break best-seller.

Three of his live, sold-out performances in Chicago have been grafted into a video, "Bill Cosby: 49" (Kodak, 67 mins., $19.95), produced by his wife Camille. It features a bundle of Cosbyisms and his perceptions of what it means to be over 40. Cosby called it "the best work about me that has ever been produced. Mrs. Cosby is the best producer I have ever worked with." Less than four months after its late-May release, more than 200,000 copies have been sold.

Cosby now has a set of videos on the shelves targeted for pre-schoolers. The set is called "Bill Cosby's PicturePages," and the first three reels are "Words and Letters," "Numbers" and "Clear Thinking" (Rainbow Home Video, 30 mins. each, $19.95). Each cassette comes with two workbooks so children can follow along. It's a warm, funny and enjoyable way to give little kids a head start on learning, as only Cosby can do.

Three more of his "PicturePages" series, "Shapes & Colors," "Animals" and "Sights & Sounds" (also 30 mins., $19.95 each), are due for release around Christmas.

When Cosby asks kids to draw a line linking three chairs and the number 3, even an adult can be tempted to pick up a crayon and join in the fun. He has that way of both commanding attention and being silly, as parents and grandparents often attempt to be when they try to engage the tots. That's probably why he has garnered so much success with his ventures on children's television programs.

Since Cosby became the first black to co-star in a non-comedy series ("I Spy," 1965-1968), many of his hits have been pegged to kids. He was Chet Kincaid, high school gym teacher, in "The Bill Cosby Show" (1969-1971). From 1971 to 1976 he was a regular on PBS' "The Electric Company," and starting in 1980 he became a contributor to "Captain Kangaroo." From 1972 until 1984 he was the honcho of "Fat Albert and the Cosby Kids."

On the inside cover of all "PicturePages" videos, Cosby says, "I am not a psychologist or a sociologist. I do have a doctorate in education, but much more important than my doctorate is my delight in kids. I devote a part of my professional life to entertaining and educating them. I like children. Nothing I've ever done has given me more joys and rewards than being a father to my five. 'PicturePages' are the epitome of what I believe in ... educating children with love and laughter. Together, we'll help your kids develop important learning skills ... like following directions, drawing, hand/eye coordination, clear thinking and numbers. Share these important lessons with the kids that enrich your life."

No surprise that in the credits of the videos one of the three educational consultants is "Dr. William H. Cosby Jr." And the jackets are clearly marked: "Endorsed by the National Education Association."

But then, Cosby doesn't really need endorsements. His track record shows that if his name is on it, it's top of the line.