EVERY WEEK for the last month and a half a member of Book World's staff has spent several days calling bookstores all over metropolitan Washington, dutifully jotting down figures and comments and titles and authors, hunched over an expanse of green accounting paper - Bob Cratchit with a pocket calculator.

The result of all this enumeration and cogitation has been the Washington list of best sellers, which has surprised many people who thought they knew just what hardcover books Washington readers would buy. The Book World staff has been surprised occasionally too, but there is nothing very mysterious about the way it has been done.

At present a sampling is taken of about 15 major booksellers in the District, Maryland and Virginia. The sampling is changed frequently, but slightly, and is growing steadily to include an "A list" of stores called every week and a "B list" of shops called only occasionally. Eventually most general trade bookstores and book departments in the metropolitan area will have been consulted at one time or another.

The stores in the sample are asked what hardcover books, fiction and nonfiction, have sold the most copies over the preceding week. Where possible, exact sales figures are recorded; elsewhere the store manager simply ranks the sales comparatively on a scale of one to ten. This indicates not only what books are selling best in what areas, but what the comparative volume of sales is on each store's individual list. (In most shops last week, for instance, Roots was selling 5 to 10 times more copies than the second title on the list, and in one case it was selling 20 times as many.) All this information is weighed, sifted and summed up, and the result is the list.

The Washington List - any bestseller list - seems to evoke strong emotions: publicists and advertisers with books on it quote its findings as gospel, authors who fail to appear on it grumble about popularity contests that have nothing to do with quality. (There were rumors last week that a major publisher is about to be sued because its lawyers cut out the juiciest parts of a gossipy memoir and thus hurt the book's chances of getting on the national lists.) Escalators in book contracts that can be worth tens of thousands of dollars are often tied to the position and endurance of a book on the national lists. In fact, several of the huge contracts that are reported shrink considerably without such provisions.

The Washington List was intended to be, and has been, a source of discovery, destroying some myths, revealing some local idiosyncracies, and providing a broad picture of reading trends among Washington residents.

One hears from time to time and far too often that only a "Washington book," heavy on power and politics will sell in Washington, or, conversely, that a Washington book will only sell in Washington. But what really is a Washington book? Over the past several weeks books that everyone has been calling Washington books, like Blind Ambition and The Right and the Power, have trailed on the list behind other more subtle literary fare. Letters of E.B. White and the novel October Light, most conspicuously, have done very well indeed.

While Trinity has dominated the fiction list and Roots the nonfiction (although ABC called it a novel for eight consecutive nights), most of the List has proved a highly volatile reflection of this area's passions and curiosities: after Mikhail Baryshnikov's much-herald engagement at the Kennedy Center a great many Washingtonians bought his expensive picture book at their local stores, earning it the number 10 slot on the list in the week after his "Nutcracker" closed; the week before Jimmy Carter's inauguration, Zero-Base Budgeting and On Being a Christian made their first appearance on the List.

Though some titles that have done well nationally have never made it onto the Washington list (Erma Bombeck's The Grass is Always Greener Over the Septic Tank, for instance), the local list has frequently foreshadowed trends on the national ones, partly because it reflects more current data.

This and a lot of other information that becomes apparent to the people putting together the list - what is selling in the suburbs as opposed to the city; why a certain title may have leaped onto the list while another plunged off it - has not been evident on the list itself. But as of this week we have revised the format to include brief descriptions of the books, when they were reviewed by Book World, where they stand on the Publishers Weekly national list, and an analysis of the various quirks, trends and curiousities encountered in the compilation of the week's list. For good measure, under the heading "Current and Choice," there will also be a number of recent books the editors of Book World have particularly enjoyed.