"ENGLAND invented the children's book as we know it," Maurice Sendak once stated in a lecture at the Library of Congress, later explaining that he had specifically been referring to the picture books of Randolph Caldecott in which "words are left out and the picture says it, pictures are left out and words say it. . . . It's like a balancing ball, it goes back and forth. And this, to me, is the invention of the picture book."

No one in our own time has as masterfully continued and extended the Victorian tradition of children's picture books as Sendak himself. "I live inside the picture book," he once said. "It's where I fight all my battles, and where, hopefully, I win my wars." And in his greatest works in this genre -- Where the Wild Things Are (1963) and In the Night Kitchen (1970) -- Sendak depicted two of his most memorable child heroes, who deal with their troubling emotions by taking two fantastic journeys: Max, punished for his mischievous behavior by being sent without dinner to his room that mysteriously turns into a forest, travels to the land of the Wild Things where he leads his subjects in three wordless double-pages of estatic dancing, only to feel homesick and to return to his room where he finds his dinner waiting. In the Night Kitchen's Mickey, on hearing the bedtime commotion of his parents downstairs, falls naked through the night into the Oliver Hardy bakers' dough, which is kneaded and pounded into a Hap Harrigan plane. Mickey then flies over the city, dives into a giant milk bottle, and finally slides back into his bed to sleep.

"It comes from the direct middle of me," Sendak once wrote to a friend about the creation of Night Kitchen," and it hurt like hell extracting it. Yes, indeed, very birth-delivery type pains, and it's about as regressed as I imagine I can go." But in his newest picture book Outside Over There -- which he considers the third of the trilogy that began with Wild Things -- Sendak has imaginatively gone even further into the past.

Based on a 200-word Grimms' fairy tale about goblins who steal a child and lay a changeling in its place, Outside Over There tells of a little girl named Ida who turns briefly away from her infant sister, whom she is tending, in order to play her Magic Horn. But on turning around she discovers that goblins -- seen first as disembodied, faceless wraiths in cloaks and later as five naked babies -- have substituted a changling for the real baby and have hidden her away. Ida flies backward out of the house in a yellow rain cloak, then switches directions and finds her sister in a cave where she plays a frenzied jig to make the goblin babies dissolve into a "dancing stream." She then rescues her sister and takes her home.

But the plot of the book is not what first attracts one's attention. Unlike Wild Things and Night Kitchen -- which drew their inspiration from King Kong, Laurel and Hardy movies, and the cartoons of Winsor McCay -- Outside Over There presents a series of extraordinarily intense, luminous, almost frozen-in-time illustrations that reveal the influence of such visionary 18th-century German painters as Caper David Friedrich and, especially, Phillip Otto Runge. In Outside Over There, the simultaneous presentation of different moods, characters, and actions -- we see the goblins planning and performing their kidnapping while Ida is obliviously taking care of her baby sister -- testifies to Sendak's compositional brilliance. The double-page illustration showing Ida, in search of her sister, flying backwards seemingly over and through inner and outer worlds, is one of Sendak's greatest achievements, while the muted but rich colors of grass, trees, sunflowers, moons and skies are the means by which Sendak conveys the story's emotional content.

As Ida returns home with her sister through a wood, we notice a little cottage in which we see in silhouette the seated figure of Mozart, whose presence betokens his role as the muse of the book. For Outside Over There is, above all, an homage to the composer who is probably Sendak's greatest artistic inspiration. ("My book is my imagining of Mozart's life," Sendak says). One can, in fact, almost hear the Queen of the Night's impassioned first aria from The Magic Flute as we see the skies darken outside Ida's window while she watches the changeling baby dissolve into water. And it was for a production of that opera by the Houston Opera (a production that will be brought to the Kennedy Center in the fall of 1981) that Sendak created costumes and sets (caves appear in both opera and book) while he was working on Outside Over There.

We should not overlook the fact that this book is also, of course, echt -Sendak. With her pluckiness and indomitability, Ida might well be an incarnation of Rosie in Realty Rosie; Ida and her infant sister remind us of the dog Jennie and Baby in Higglety Pigglety Pop!; Ida flying resembles David flying in Fly By Night; and Ida's rescue mission parallels Max's and Mickey's night journeys. But while Max contacts his feelings of anger and loneliness as he travels to the land of the Wild Things, and while Mickey explores the feelings of his unrepressed body as he journeys in the world of the Night Kitchen, Ida and her sister experience feelings of loss -- the loss of identity and dissolution -- only to recover both themselves and each other.

In Outside Over There, it is as if Sendak, in the character of Ida, had returned to the first moments of conception -- Ida's ritual encounter with the goblin infants just out of their eggshells hints at this. And it is interesting that in this book, Sendak has given us the first female protagonist of his trilogy (in fact, the goblin babies, too -- and everyone else in the book except Mozart -- are female). It is as if he had contacted the realm of his anima -- in the Jungian sense of the female side of a man's nature. One can make such speculations because Outside Over There, like most great fairy tales, has the simplicity of an elemental story and at the same time the mysteriousness, the depth, and the multiplicity of meanings of a dream . . . as we, like Ida, enter the underworld of the goblins' cave, where what is outer becomes inner, and where what is lost is found.

Outside Over There is the first of Sendak's works to be published and distributed as both a children's and an adult book. This is as it should be, for Sendak has always had the uncanny ability to make us, as adults, reexperience the way a child experiences his or her earliest emotions, reawakening in us our own childhoods. As Runge once said: "We must become children again if we wish to achieve the best." And in Outside Over There, Maurice Sendak has achieved the best.