Masterful photo essays, some never published; W. Eugene Smith: Master of the Photographic Essay, edited and with commentary by William S. Johnson. Millerton, N.Y.: Aperture. 223 pp. $39.50.

''W. Eugene Smith was famous at twenty and a legend by forty,'' says William S. Johnson in his introduction to this extraordinary collection of photographs. And how right he is - Smith is surely the finest photojournalist of our time, and this book is the proof.

Smith is probably best known for the many exceptional photo essays he did for Life magazine between 1946 and 1954, when he went on to pursue a variety of free-lance projects that occupied him until his death in 1978.

Indeed, it is hard to think of finer photo essays than ''Country Doctor'' ( 1948), ''Spanish Village'' (1950), ''Chaplin at Work'' (1952) and ''A Man of Mercy'' (1954), his moving study of Nobel Prize winner Albert Schweitzer at work in Lambarene, West Africa.

But these photo essays constitute only a small portion of Smith's large and exemplary career, a career that began in Kansas, when Smith photographed local sports for the Wichita Eagle while still a high school student.

Smith realized very early exactly what direction he wanted his life to take. Near the end of his first (and only) full semester at the University of Notre Dame, Smith, then 17, wrote his mother:

''My station in life is to capture the action of life, the life of the world, its humor, its tragedies, in other words, life as it is. A true picture, unposed and real. . . . I want (my pictures) to be symbolic of something.''

Heady stuff for a 17-year-old, yet Smith made it all come true. By the age of 19 he was working for Newsweek, at 20 for the famous Black Star photo agency, and then for Life.

In the late 1930s and early '40s Smith published numerous photo essays in Life, Colliers, Parade, American Magazine, and Woman's Home Companion.

When World War II broke out, Smith worked first as a correspondent for Flying magazine (1943-1944), and then for Life. At Okinawa, Smith was on the front lines with the troops and took such risks that his fellow photographers began, says Johnson, ''to fear for his safety.'' In May of 1945 Smith was wounded by shell fire. This ended his career as a combat photographer.

After the war was over, Smith returned to Life magazine and between 1946 and 1952 photographed roughly 50 assignments for that publication.

The strength of this monograph is that it reproduces not only the essays for which Smith is famous, but many of his photo essays that were never published and which we now see were as good as his published work.

''W. Eugene Smith'' contains over 1,800 photographs -- an unusally large number of images for a monograph -- because each reproduction is roughly two by three inches. The occasional frustration that the small size of the images causes is more than made up for by the sheer mass of images. Photography is, after all, a cumulative art.

It is difficult to praise this collection highly enough and, indeed, to cover adequately the many achievements of Smith's career. If there is one book people interested in photojournalism should have, this is the one.