Charles Merrill's ''The Walled Garden: The Story of a School'' is a complex and rewarding essay. It concerns the Commonwealth School in Boston, which Merrill founded in 1958 - a private, secondary day school fashioned out of adjoining row houses on the corner of Dartmouth and Commonwealth Avenues. It's a school whose single rule is ''Don't roller skate in the hallways,'' a school in which one finds possibly the most brilliant faculty and student body of any high school in the land, a school from which students with startling credentials go out to the most prestigious of colleges and universities.
Like the school, which includes the book's central metaphor, ''The Walled Garden'' is fascinating - and appropriately difficult to categorize. On the surface it appears a brisk narrative reminiscence. The first chapters summarize family history: Merrill's education at Deerfield and Harvard, his service in World War II, secondary school teaching in St. Louis, and two years in Paris as an unpublished novelist. Returning to Boston to start his school, he ''bought a huge decaying rabbit warren on the corner of Dartmouth and Commonwealth, because it had a lot of space and a stained-glass window in the room that would be my office.''
The essay proceeds chronologically from Commonwealth's beginnings, through the antiwar protests of the late '60s and early '70s, and on to Merrill's final years at the school and the search for a successor. It ends, 23 years later, with his cleaning out that office with the stained-glass window.
To leave it at that, however, would be as inaccurate as calling ''Seven Pillars of Wisdom'' the ''story'' of Arab skirmishes early in our century. Merrill's explicit chronology is presented in four untitled sections. They could be titled Starting the School, Troubles, Efforts to Cope, Winning Through - or Vision, Reality, Individual Suffering, Meaning. Merrill presents a pattern, but leaves the naming of parts to us. A tour de force of provocative juxtapositions, the book is laced with allusion: The reader must handle references to everything from Miss Havisham's cake to John Lukacs.
And be alert to its frequent, pervasive irony. When Merrill says he bought his rabbit warren ''because'' it had space and a stained-glass window, the word ''because'' is held up to the light and twisted about. It brings to mind Hardy's poem ''The Man He Killed,'' in which the narrator killed his enemy because . . . well . . . ''because he was my foe.'' Just so. Behind explicit statement of cause and effect lurks a question: How can we ever really know the cause of what we do? ''I tend,'' Merrill says, ''to think that no real solutions exist to any questions of importance.''
Thus anecdote constantly becomes the occasion for discussions of unanswerable questions. Why war? What does it mean to be educated? What is good? What power has reason in a world beset by irrational violence? Why must we suffer? The text makes clear that if a school is to take the real risks its founder laid out for it, it cannot maintain the ''walled'' metaphor. To become educated, to become attuned to the great and unanswerable questions, is to challenge the morality of the garden and set about breaking down its protective walls.
This challenge elicits one of the most interesting elements of the essay, which is that new images are to be found in Europe. Merrill wanted his school ''to attain the intellectual distinction of the European schools at their best.'' Villandry, a 16th-century chateau in the Loire Valley, provided the literal image for the figurative walled garden in Boston. The resiliency of European culture, with its images of dignity in the face of suffering, pep up the headmaster weary with his role as cop and with the solipsistic outlook of his students. He begins his book, in fact, with a European image: a wonderfully rendered story of an American soldier in Italy - a Good Soldier, the Doppelganger in the essay - simply doing his job repairing radio-phone wires on the slopes of Monte Rotondo while Americans and Germans blast away at one another.
Merrill clings to the image of this good soldier - doing his job, doing it well, repairing lines of communication in a world beset by irrational strife. Theodore Sizer, in his introduction, finds Merrill ultimately optimistic: ''. . . His faith in man's mind overcomes.'' While I do not hear that optimism and find the concluding imagery of the book very sad, it is testimony to the writer's power and complexity - and his refusal to peddle easy answers - that ultimately the tone and meaning remain open questions.
Certainly ''The Walled Garden'' should, as Sizer suggests, be read by those interested in school reform. It should be read, as well, by anyone who enjoys thoughtful and challenging prose, rendered with grace and wit, about questions that continue to tax us all.