A reader -- a vexed, articulate, thoughtful reader -- checks in as follows:

"What I cannot figure out for the life of me is why . . . The Post wastes its space reviewing such tripe as 'Black Widow.' You denounce the book as one which was 'published solely to make money,' and as 'a striking example of what the American publishing industry can come up with when it is at its most cynical and manipulative.' You then conclude by wondering 'why it makes money.' And yet you review the book! Why?"

This gentleman goes on to remark that there are countless "serious books" that have not been reviewed in the Post. "Unlike 'Black Widow,' " he writes, "they have no multimillion-dollar advertising campaigns to rescue them from reviewers' ridicule or inattention. The publishers must shoulder part of the blame for this, but if The Washington Post can't refrain from wasting a thousand words on such trash as 'Black Widow,' what right have you to complain about the publishers?"

Good questions and important ones -- ones that are frequently raised, in various ways, by serious readers of books and book reviews. It is impossible to answer them to anyone's complete satisfaction, because everyone has different interests and expectations. A newspaper is in many respects the product of compromises; in trying to meet the interests and expectations of as many people as possible, a newspaper invariably disappoints or angers others as it goes about its business. When it comes to the reviewing of books, anger and disappointment seem to be constants.

In substantial measure this is because book reviewing is fundamentally different from any other kind of reviewing that a newspaper performs. In reviewing the theater, the movies, the dance, the art galleries -- virtually everything else -- the newspaper doesn't have to waste much time fretting over what to review. It reviews just about everything: all the major dramatic productions and most of the minor ones; all the movies except pornography; all the art shows except the occasional one that escapes its attention or is inadequately publicized. There is a limited number of shows and productions, and the newspaper has sufficient space to acknowledge the existence of just about all of them.

But books are another matter altogether. Some 40,000 trade titles are published each year in this country. The Post's Book World, daily and Sunday combined, has space to review about 2,000 of them -- a figure that includes very brief reviews of paperbacks, mysteries, science fiction, children's books and Christmas gift books. Of course, a great many of the 40,000 are books a newspaper is not expected to review: scholarly monographs, specialized publications for limited audiences, how-to books, what have you. But even if you reduce the number of legitimate candidates for review from 40,000 to, say, 20,000, it still means that Book World can review only one of 10 books competing for its space.

Choices, in other words, must be made, and choices can be tough. Since the subject has been raised, let's look at the case of "Black Widow," which I reviewed in The Post on Dec. 9 and which I found distasteful and contemptible on all counts.

In a just world, "Black Widow" would not have been reviewed; it would not have been published or even written. But in the real world in which we live, it came into these offices not merely as just another new book. Christina Crawford, its author, was previously the author of "Mommie Dearest," a vituperative attack on her mother, the late actress Joan Crawford, that was a huge best-seller and is now a commercially successful movie starring Faye Dunaway. A new book by Christina Crawford is, like it or not, going to be of great interest to a great many readers. They bought or borrowed her first book; do they want to buy or borrow her second?

Enter the newspaper. The decision to review "Black Widow" was in point of fact an easy one, because the book was news. Unlike a literary journal, a newspaper cannot afford to set up shop in an ivory tower and look down with contempt upon the base world of commerce. As a matter of journalistic responsibility it has to cover the bad books as well as the good ones -- and it may in some cases have to cover the bad books instead of the good ones, if the bad ones are clearly newsworthy.

It can further be argued, in response to my correspondent's questions, that a positive purpose is served by devoting several paragraphs to an attack on a book as sleazy as "Black Widow": The review may actually persuade a few people not to purchase the book. Though the power of book reviews should not be overestimated -- if readers followed the advice of reviewers, James Michener and James Clavell would not be wealthy men -- there seems at least an outside chance that a review as negative as mine may have steered a reader or two away from that book. In the case of "Black Widow," I regard the exertion of any such influence as a clear public service.

Still another point. There is in my correspondent's letter a rather explicit argument that it is the responsibility of a newspaper's book-review section to act as defender and promoter of "good" and "serious" literature. Holding my breath, I beg to disagree. It is the job of a book-review section to review books. Obviously in passing positive and negative judgments on them, a review section acts as what a forthcoming study of the publishing industry calls a "gatekeeper," printing opinions that may help shape a book's future and its author's reputation. But a comprehensive and responsible book-review department must pay attention to the full spectrum of trade publishing, not merely to the authors and publishers whose work is approved by the English departments, the literary salons and the small bookstores.

What needs to be emphasized, again, is that we are talking about a newspaper, not a literary journal. The New York Review of Books, with a circulation of some 100,000 certifiably intellectual readers, can limit itself to the books it knows to be of interest to them; that is why it has made a considerable success against considerable odds. But the nearly one million households into which The Post's Sunday Book World is delivered are as diverse as the community itself; in choosing which books to review we must take that diversity into account, which means recognizing that our readers are interested in "commercial" books as well as "serious" ones.

What's true for The Post is true for every newspaper of general circulation -- except that in reviewing 2,000 new titles a year, we cover more books than all but one or two papers in the country. Because we do, we are -- in point of fact -- able to review a great many of the serious books that my correspondent is so anxious to see publicized. A week after reviewing "Black Widow," I reviewed Aharon Appelfeld's "The Age of Wonders," a masterly novel published by a small and still-struggling press -- and the review was displayed much more prominently than was that of "Black Widow." On the first Wednesday of this month I reviewed a book published by a university press, and on the last Wednesday I will review another; three of this month's five Wednesday reviews, in other words, will be of books issued by "serious" publishers.

Perhaps reviewing "The Age of Wonders" is taking care of art, and reviewing "Black Widow" is taking care of business. But if that is so, then that's life. There's room in these columns for both art and business, and that, in my judgment, is as it should be.