ONE PAPERBACK this season seems clearly designed for summer reading: The Serial: A Year in the Life of Marin County, by Cyra McFadden (Knopf, $4.95) is spiral-bound, so that you can shake the sand out of the pages with a minimum of effort.

As for what's printed on the pages, The Serial is a trendy new newspaper soap opera that chronicles the very closely interlocked lives of a group of Californians: the cliches they substitute for ideas and conversation, the fads in food, clothing, furniture and sexual arrangements that sway their ranks like an overripe cornfield, the all - too - credible adventures of their ghastly off - spring. The style (a sort of archetypal Hardee Mumms) is redolent of with - it - ness, brand - name chic and fresh - minted jargon ("I' can relate to where you're coming from") that sounds shopworn before the ink is dry.

The Serial represents (in rather extreme form) one theory of the summer paperback: total frivolity to dispel the noxious humidity or to fill the empty hours at the beach. It predestined readership is large and avid, in contrast to the book itself, which is short and (in its own terminology) laid back.

At the opposite end of the spectrum is another theory of summer paperbacks, suited to highly motivated readers: the summer, with its wide - open stretches of free time, is an opportunity to do all that serious reading you were too busy for in the winter. For example, those really important books being prepared bythe Council on Foreign Relations in connection with its 1980s Project: titles such as Nuclear Proliferation: Motivations, Capabilities, and Strategies for Control, by Ted Greenwood, Harold A. Feiverson and Theodore B, Taylor (McGraw - Hill, $4.95).

There are those who feel that summer is a logical time to read about summery things, and you may see them browsing through the serried ranks of statistics (more than 600 pages of them) in The Official Encyclopedia of Baseball: Ninth Revised Edition, by Hy Turkin and S.C. Thompson, revisions by Pete Palmer (Dolphin, $5.95; Doubleday hard - cover, $14.95). Or How to Redesign Your Yard and Garden, by Stanley Schuler (Hawthorn, $4.95).

For some, summer is a time to indulge in those long, epic novels that seem to be impossible to stay with during the busier seasons. The two - part Poldark saga, for example, which is now being shown on television: Ross Poldark and Demelza, by Winston Graham (Ballantine, $1.95 each). There is all the difference in the world, of course, between a book based on a television series on a book on which a television series is based.

If your attention span slackens in hot weather, you want something brief, though not necessarily insubstantial. Perhaps All Our Sercrets Are the Same: New Fiction from Esquire, edited and with a foreword by Gordon Lish (Norton, $5.95), which belies splendidly the idea that the short story is a dying form. The writers are among the best authors of fiction in our time: William Kotz - winkle, Milan Kundera, James Purdy, Cynthia Ozick, Grace Paley and Don De Lillo, to name a few.

Come to think of it, any paperback can be a summer paperback, from John McPhee's beautifully written The Survival of the Bark Canoe (Warner, $4.95) to Rural Society in France: Selections from the Annales, Economics, Societies, Civilisations (John Hopkins $3.50: hardcover, $12.50, As long as you don't mind getting a bit of sand in between the pages.