IN THE BEGINNING WAS ONE Britannica pedia. Then, in the 15th edition of the world's most prestigious compendium of information, knowledge trifurcated into a propedia, macropedia and micropedia. Now we have a series from a new collaboration by Britannica and Bantam that can only be called a megalopedia.

According to the series editors, these volumes draw upon such sources as "the mine of material in the 14th Edition, a great work in its own right, which is no longer being published because much of its material did not fit the design imposed by the 15th." Are we to take it, then, that the series is a restoration of knowledge left out of the new, polypedic Britannica, a Remembance of Knowledge Lost? In any event, each volume is a comprehensive survey of a broad topic, something like a college 101 course in softback.

Broadest of all is the volume called Energy, which stretches its rubric to cover coal gastification, the Big Bang theory of universal origin and everything in between. This admirable book is especially strong in its ability to render abstruse comological concepts in comprehensible terms. Here, for example, is as succinct and clear an explanation of the astronomical notion of the black hole as the layman can find anywhere:

"The Marquis de Laplace, an eighteenth-century French astronomer and mathematician, had reasoned that light, having mass, would be subject to the force of gravity. If a star were massive enough, gravity would prevent light from escaping into space. Gravity would enclose it like the shutters of a dark lantern. That could reconcile latterday scientists to the presence, in space, of gravitational locations that should have been associated with a massivee body. By Laplace's definition, these were black stars. The remorseless logic of the black hole, however, goes much farther; it pursues matter into a physical nirvana or total oblivion. All that is left, like the smile of Lewis Carroll's Cheshire cat, is a measurable gravitational force where the vanished star once was."

The writers of this passage have skillfully combined pedagogical tools in this passage: summoning an historical figure who propounded an early, simplified version of the current theory; resorting to nonscientific images and terms, such as "shutters" and "nirvana"; alluding to Alice in Wonderland ; and writing in a generally sprightly way.

Now and then the editors strain themselves in their quest for the sprightly. One chapter begins: "The sun is a middlesized, middle-aged, middle-class star, existing in an outer suburb of the Milky Way" -- a description that makes the sun sound like a subscriber to TV Guide. But with the arcana of intergalactic physics on the agenda, one cannot fault Britannica/Bantam for erring on the side of chattiness.

The volume of American government is nearly as good, merging comprehensiveness with some nice insights into the workings of the system. It is useful, for example, to be reminded of how seldom the Constitution has undergone amendment. The 10 amendments that form the Bill of Rights came as a lagniappe with the document itself, the 11th and 12th Amendments are technical, and the 21st merely cancels the 18th (the Prohibition Amendment). This leaves only 13 substantive amendments since 1789. And anyone who values eccentricity will be pleased to learn that, despite its electoral insignificance, the Anti-Masonic Party contributed its national convention method to both the Republican and Democratic Parties. (For the record, the anit-Masons reached their acme of direct influence when they carried Vermont in the presidential election of 1832.)

There is scarcely a subject more in need of demystification than the law, but the Brittanica effort here is not on a par with the rest of the series. The text is abstract and tabular. Of what value is a list of all the Supreme Court Justices and their dates of tenure when so few of the cases they decided are discussed? (Many are mentioned, few are discussed.) And was it really necessary to reprint those civics-book pie charts that show where our tax dollars come from and go?

The editor's slant on the big issues is sometimes disappointingly standard. To the layman, one of the most disturbing aspects of the legal profession is the ethical principle that allows a criminal lawyer to defend a guilty client. By way of explanation, the editors quote from the American Bar Association, which supports the principle because "'Otherwise innocent persons, victims of only suspicious circumstances, might be denied a proper defense.'" But this rationale weasels out from under the premise: We all know -- and any candid defense attorney will admit -- that lawyers sometimes defend to the hilt people who are unmistakably guilty. The justification, it seems to me, is that the guilt of even the bloodiest black-guard must be established by means of orderly and fair procedures; otherwise, the legal system slackens to the detriment of us all. A good way to insure that the system does not degenerate is to assign partisan experts -- lawyers -- the task of insisting that it always operate fairly and above board, no matter how loathsome the defendant. A scrupulous lawyer can vigorously defend a damnable villain, then, not so much for his sake as for yours and mine.

On the whole, this is a worthy new series. Worthy enough, in fact, to warrant our overlooking its role in Brittanica's reckless proliferation of pedias.