What makes American farms work; Feeding Multitudes -- A History of How Farmers Made America Rich, by Wheeler McMillen. Danville, Ill.: The Interstate Printers and Publishers (19-27 North Jackson, 61832). 491 pp. $12.95.

The growing gulf of understanding between the 1.5 percent of the US population who are food producers and the 98.5 percent who are consumers makes Wheeler McMillen's cogency about our farms and ranches far more essential reading than the current best sellers about cats and diets.

Longtime editor of Farm Journal, McMillen shrewdly avoids polysyllabic jargon. He spins droll anecdotes to illustrate the basic mores of the land and its caretakers. The 73 chapters are organized into sections that succinctly narrate Beginnings . . . Fundamentals . . . Energy . . . Knowledge . . . Business, etc. Thus emerges the best mosaic yet of how ''the man with a hoe'' and ''the lonesome cowboy'' were pressured by machines and government edicts to become a sophisticated class of businessmen.

Especially useful are appendixes titled People and Dates. People is 44 pages of brief biographies of the men and women the author deems ''most outstanding'' in the 374 years of American agriculture. Dates is a 25-page listing of that era's most significant events.

''Feeding Multitudes'' is more than good reading and good learning. It deserves shelf space as a permanent reference on the foods we live by, the valiant individualists who produce them, and the future of our ''most lavish diet on Earth.''