"The Cosmos is all that is or ever was or ever will be," Carl Sagan declares, as though he had been there and back. "The surface of the earth is the shore of the cosmic ocean . . . . The ocean calls. Some part of our being knows this is from where we came. We long to return."

From this launching pad, Sagan rockets off on a personal tour of the universe, skipping through a lesson on the principles of evolution, speculating about life on other planets, giving a short history of astronomy, attacking the cult of astrology, describing the great comets, summarizing the recent explorations of the Moon, Mars, Venus, Jupiter and Saturn. It is sometimes hard to discern a plan in this cosmic trip, but the guide's comments are always informative, amusing and purposeful.

Sagan is an enthusiast, and that may be why many of his fellow scientists regard him with suspicion. He is passionate about his insights, and passionate about sharing them with the rest of the world. His glib style and self-absorption are at times annoying. Professional scientists, who are leery of popularizers in any case, certainly are skeptical. Although Sagan has distinguished himself as an astronomer at Cornell University, his peers are likely to see in this book the confirmation that he is a promoter rather than an originator of ideas, and an eccentric one at that. They may also envy his tremendous success.

Nonspecialists obviously are pleased to find someone from the elite who is willing to synthesize the new facts churned out by 20th-century science and hammer them into a comprehensible story. Sagan is perhaps the most ambitious of the synthesizers. He has published popular books on the evolution of the brain and the development of scientific thought ("The Dragons of Eden," 1977, and "Broca's Brain," 1979), speculated about nonterrestrial life ("The Cosmic Connection," 1973), tried to interest the government in contacting intelligent beings in space, and made himself famous as an interpreter of natural mysteries. Now, with a major television series this fall on the Public Broadcasting System and a Christmas book to go with it -- both called "Cosmos" -- he has plainly transcended his academic beginnings. He has become not so much an entertainer as a prophet of science.

"Cosmos" is a bit odd for this reason. Beautifully illustrated, and written in an engaging style, it is sustained by something unexpected: an inspirational message. The result is a kind of gospel without deities. Its guiding insights are fairly straightforward. Life is the supreme treasure, followed by human knowledge. The universe is beautiful and well ordered. Deciphering its laws should be the chief goal of mankind. Human nature threatens to distract us from this goal and destroy the small measure of life and knowledge we possess. Good people should rise up and insist that the nations stop their warmaking and turn to their telescopes. This is not a surprising new message, but it is unusual to find it put forward so passionately by a scientist.

This is a good thing, I believe. First I think that Sagan's message is preferable to those offered by other mass inspirationalists like Sun Myung Moon, Jerry Falwell, L. Ron Hubbard and the ayatollahs. Perhaps it will give them some rationalist competition. (The first show in Sagan's television series proved the most popular ever on PBS.)

Second, by speaking with a personal voice, Sagan lends his work a resonance and coherence it would otherwise lack. It provides the thread that connects the disparate sketches of ancient astronomy, radio telescopes and fourth-dimension mathematics. Even the planets, with their dreary landscapes and poisonous clouds, speak to Sagan in human terms. "The surface environment of Venus is a warning," he writes, "something disastrous can happen on a planet rather like our own." He hopes we take note and control our industry before it is too late.

The weakness of this approach is that the story depends heavily on the special insights of the narrator, and this may be why the tale takes some odd turns. (Did yhou know Johannes Kepler died in a town called Sagan?) In a sense, one must like Sagan as well as the universe to enjoy this cosmic tour. I found the guide to be a little overbearing, but still informative and entertaining.