THE BEST AMERICAN ESSAYS 1988; Edited by Annie Dillard; Ticknor & Fields; 328 pages; $17.95.

The essay, that hardy literary form that continues to survive despite years of neglect, seems to be on the rise.Numerous books of essays have been published in recent days, and this collection of 20 is among the latest. Why the essay is making a comeback isn't known for sure, but one reason may be the versatility it offers writers.

As editor Annie Dillard observes in her introduction: "The essay is, and has been, all over the map. There's nothing you cannot do with it; no subject matter is forbidden, no structure is proscribed. You get to make up your own structure every time, a structure that arises from the materials and best contains them. The material is the world itself."

And so it is. A world that contains places as different from each other as they can be, places such as holy Lourdes and the killing fields of Okinawa.

In "On the Pilgrim's Path to Lourdes," Eleanor Munro takes a clear-eyed look at the French shrine and the more than 4 million pilgrims who visit it each summer.

She tells its history and of its reputation for miraculous cures. Despite the commercialization of the place, Munro concludes that "even if the cures are dubious or short-lasting, the patients return home, sometimes to institutions that are their lifelong homes, lifted in mind and heart by the experience."

The bloody fighting on Okinawa that took place during World War II has receded into history. But William Manchester remembers it as clearly as if it were only yesterday.

He was a Marine sergeant during the 82-day struggle in which more than 200,000 perished. The United States needed the island to stage a planned invasion of Japan. Because of the atomic bomb, the invasion would never take place - but the battle for Okinawa did, and Manchester tells in chilling detail just what it was like to be there.