Provocative call for global unity, peace; Earth at Omega, by Donald Keys. Boston: The Brandon Press (P.O. Box 843, 02147). 117 pp. $13.95 postpaid.

''Earth at Omega'' isn't likely to compete on the best-seller list with Jonathan Schell's monumental study of the nuclear arms threat, ''The Fate of the Earth.'' Yet Donald Keys' book is in many ways a quiet voice that contrasts with Schell's earth-quaking prose. Whereas much of the recent doomsday literature is fearful reading, ''Earth at Omega'' is constructively hopeful. Keys accepts the lunacy of nuclear war as a given. From this premise, he takes the reader on a thoughtful investigation into what might be done about the myriad, mounting problems of the world - the nuclear threat, overpopulation, hunger, resource depletion, pollution, and others.

But Keys, a long-time UN observer and consultant, doesn't compartmentalize the world's problems, and then offer isolated solutions to them, one by one. Keys has a vision to share. In fact, he sees the problems themselves as opportunities for men, women - and nations - to realize their essential oneness and their need of developing unity on a global scale.

The title of the book, as well as much of its cosmological base, is taken from Pierre Teilhard de Chardin's concept of a ''Point Omega'' - a point in earthly history in which man must decide between extinction and survival - but not just bald survival. As Keys elaborates, the leap in consciousness required to establish peace wouldn't leave humans where it found them. Keys writes, ''What we will need to avoid catastrophe is a critical mass of people - and ultimately of nations - who have adopted goodwill as their dominant expression in personal and in international affairs.''

Although Keys' book is brief, it covers enormous territory: current parallels to the birth of democracy in America; the problems and promise of the UN; recent planetary prophets; useful archetypes; new seedlings; and, at the end, an explanation of Planetary Citizens, the grass-roots organization Keys helped to start.

There are some difficulties. The book lacks the editorial shine found in releases from the bigger publishing houses. The prose is occasionally labored. Points are glossed over. And, considering the weight of the material he deals with, some conclusions may seem too glib or easy.

Halfway through, however, one steps back and recognizes the grandeur of Keys' vision. Happily this rescues the book. Because Keys' ideas are so alive and far-reaching, they demand that we question more deeply our own standpoints. Though all won't agree with the whole of Keys' outlook, we would be shortsighted indeed not even to consider his suggestions and their implications.

''Earth at Omega'' is only an introduction to a momentous theme. But it can't fail to challenge the individual. And that, Keys says, is where planetary progress begins.