THE BEST AMERICAN ESSAYS 1990; edited by Justin Kaplan; Ticknor & Fields; 308 pages, $19.95.

In his introduction to "The Best American Essays 1990," editor Justin Kaplan poses the question, "What is an essay?"And, after examining his question from a variety of angles, Kaplan wryly notes: "All you can safely say is that it's not poetry and it's not fiction."

He's right, of course - as the 21 essays in this delightful volume prove. Ranging from such various, and unrelated, subjects as earthquake-stricken Armenia, a kind word for envy, an Elvis sighting and a wonderfully funny piece about lawns, each essay stands alone and does so perfectly.

Michael Arlen tells of his trip to see at first hand the earthquake-damaged Soviet Socialist Republic of Armenia in "Armenian Journal." Carefully researched and quite sympathetic to the victims of the quake, Arlen still manages to lighten his piece with witty observations of the Soviet airline system and with Armenian bureaucracy.

In "A Few Kind Words for Envy," Joseph Epstein does a marvelous dissection of envy in the form of a patient speaking to his doctor. He concludes: "Envy is apparently more easily felt than defined. . . . Envy, I say, is desiring what someone else has - a desire usually heightened by the knowledge that one is unlikely to attain it."

A nice lawn is the desire of most homeowners. Not Michael Pollan. Pollan went along with the "keep a nice lawn" ethic for a while. Then he decided to let his mowing machine go to rust and he most amusingly tells why in "Why Mow? The Case Against Lawns." - Phil Thomas (AP books editor)