Is there a sixth sense -- magnetic?; Human Navigation and the Sixth Sense, by R. Robin Baker. New York: Simon & Schuster. 138 pp. $14.50.

British zoologist Robin Baker has an intriguing suggestion - humans may have a sixth sense, a magnetic sense, that enables them to use Earth's magnetic field for navigation. Operating more or less unconsiously, like the sense of balance, it helps us maintain our orientation.

Such a magnetic sense has already been clearly demonstrated in a wide variety of organisms, ranging from bacteria to certain insects, birds, and mammals. Indeed, the magnets themselves have been found in many such organisms. No such magnets have yet been found in humans, as far as I know. However, Dr. Baker claims to have evidence for a magnetic sense in humans, derived from experiments with British university students in which some students had magnets strapped to their heads to confuse any sense of Earth's magnetic field while others wore inactive dummy systems.

Unfortunately for Dr. Baker's claims, similar experiments in the United States have been unable to replicate his results. Thus the evidence is inconclusive, and readers should take his hypothesis, at this stage, with reservation. But this does not detract from the merit of his excellent, readable , and informative book.

To begin with, it is an interesting summary of the fast-developing field of animal navigation. Secondly, while Dr. Baker may present an unproved hypothesis, he is thoroughly scientific in doing so. He makes no claim to definitive proof, nor does he leave readers in doubt as to the tentative nature of his findings.

Thus the book is an essay in scientific exploration -- a personal statement of how one scientist is working out new knowledge that may give people a new perspective on themselves. In this it contrasts with ''The Mystery of Migration'' (see full-length Monitor review Feb. 9, 1981), for which Dr. Baker is chief contributing editor. That book treats all aspects of migration -- not just navigation -- in many organisms, including plants.

And what does modern knowledge of navigation tell us about ourselves? That is an open question, which is only beginning to be explored. However, Dr. Baker leaves us with the provocative observation: ''if humans are the equal of pigeons in matters navigational, then likewise pigeons are the equal of humans.''