"YOU COULD SAY that what we're into is the largest promotional campaign in human history to make people interested in general knowledge."

That is how the book's editor-in-chief, James Mitchell, describes the publication of "this passionate book," The Random House Encyclopedia (2856 pp., $69.95). And Robert Bernstein, Random House board chairman, is only slightly more modest: "This book is going to open people to more subjects in less space for less money in less time than any other book that now exists."

It could happen. The Random House Encyclopedia is a happy combination of public service and probable publishing bonanza. It is also an unusual joint venture in international publishing.

Mitchell, the British publisher of The Joy of Sex , among that celebrated titles, originally conceived the plan for a picture encyclopedia in which the subjects would be arranged in logical, rather than alphabetical, order. He felt that traditional encyclopedias had two basic faults: they were grim and "passive"; and their alphabetical structure prevented adjacent items from having any necessary relationship to one another.

Mitchell wanted to organize knowledge thematically - instead of finding "food" and "nutrition" under separate headings, the two subjects would be contiguous in the text. In addition, he wanted the printed word to be visually subordinate to color art which was both so striking and so densely informative that it would communicate at least as effectively as the paragraphs of prose. But, of course, some kind of slphabetical organization proved necessary: hence the dictionary-index (and "alphapedia") to the larger color section (the "colorpedia").

The encyclopedia has refreshingly modest and humane goals. It is not intended, or promoted, as the final answer to any reference problems, although there is a significant amount of factual and conceptual information there. Bernstein is quite frank about the book's aim: "At Simon & Schuster every editor had this brass plaque on his desk.It said Give the Reader a Break . That's what we're trying to do with this book."

How much of a break the reader gets varies from section to section. Depending upon the subject matter, the style varies from compact to compressed, and the reading level is often surprisingly high. Bernstein feels that very few people are too sophisticated for the book: "I think that one of the big things wrong with this country is the way that adults overrate what they know. I can tell you that most of the adults in the United States do not know 80 percent of the material in this book."

"Really," Mitchell insists, "nobody sat down at a table and analyzed a multi-million dollar market and said 'how can we penetrate it, yak, yak, yak,' like some great corporation idiot."

But, of course, it takes a lot of "corporation idiots" to market an expensive book, and while Mitchell was wondering about the future of his idea, a great deal of the old "yak, yak, yak" was going on at Random House.

Robert Bernstein remembers: "We had felt, ever since we brought out The Random House Dictionary [1966], that we could eliminate a lot of the things that people didn't use - and therefore didn't have to pay for. And after we finished the dictionary, we felt the same thing about encyclopedias, that there was a tremendous need for a reference book that could go right into the home, that wasn't prohibitively expensive, that could fill an enormous number of needs - and that if we could design such a book, we would have an enormous market."

Others agreed. The core of the encyclopedia - which Mitchell is publishing in England in 10 volumes entitled The Joy of Knowledge - will be issued by publishers in 13 countries as diverse as Nigeria and Finland.

Mitchell's original opus had to be thoroughly Americanized in both language and subject matter, Random House was not interest in publishing an international encyclopedia . Such parochial gratifications as maps of the individual states were added, and a team of editors rooted out potential male chauvinism in the presentation.

The joint effect which resulted is one sturdily bound volume printed on first-quality paper and designed to sit on a table and get a lot of use. Its nearly 3000 pages (including the obligatory 80-page color atlas and a "Time Chart" covering the period from 4000 B.C. to the present in 50 pages) weight about 12 pounds and are four inches thick. The first 1822 pages contain 875 two-page color "spreads": two facing pages which treat a single subject. Two-thirds of each spread is devoted to pictures, photographs, diagrams and charts. the remaining third contains an average of 1450 words. The formate is obviously somewhat restrictive, since no subject is allowed to run over two pages. Consequently, the same space is allotted to "Global Tectonics" and to "Green Salads": "Judaism and Christianity" get two pages, and so does the "HIstory of Bicycles." But nobody claims that it's the Columbia Desk Encyclopedia, a more traditional and academic reference with no color - strictly aphabetical but containing a lot of information. "In academic terms, the Columbia probably has more information." Bernstein admits, "but then, we're not competing with anybody - ours is a wholly new institution."

But if the spread concept is reductive, it is also seductive, drawing the reader into the material, and the 1450-word ceiling on the prose block keeps the reader from feeling that he had gotten into something that he cannot finish. This is a powerful incentive to stay with it, especially for younger readers, in a generation whose reading attention-span may not be longer than a commercial break in Kojak .

It would be easy to conclude, looking hastly at the book, that Bernstein and Mitchell had set out to exploit the new inattention of the American reader, capitalizing on the People magazine success formula of short text and long pictures. It would be equally easy to condemn the book as a concession to the television age, as a McLuhanish melange of pictures and factoids which give the illusion of learning without the substance.

Those conclusions, however, would be wrong. The publishers are aware of the changing needs of the television generation, and can draw on batteries of psychological texts which demonstrate how rapid eye movement among carefully arranged blocks of color apparently contributes to enthusiastic learning.But it is impossible to talk with Bernstein or Mitchell without feeling their genuise sense of purpose.

"It's the job of publishers to have an optimistic view about human nature," Mitchell says. "We've got to believe the people want to know. If you believe that they have an innate curiosity, an innate desire to improve their knowledge and expand their horizons, then the game becomes how best to find to form to interest them.

"That form changes from generation to generation. And the change that has been recognized by Random House as a publisher - and aided by me scurrying around as an enthusiast - is that if you want to turn people on, want to capture their imaginations, you're going to have to work a whole lot harder."

Judged in those terms, the book works very hard indeed. And even sophisticated, skeptical consumers will have to admit that no book on the market can do what The Random House Encyclopedia does best: to provide what Mitchell calls" a tremendously informative and rewarding browse." "We're not trying to give anyone the last word on a subject," Bernstein says. "What we want to do is give them the first word."