They're arriving already, hitting doorsteps before the first leaf falls or the last crocus blooms: Christmas catalogues, bearing good tidings of great gifts from the $20 billions-a-year mail order catalogue business.

A $600 edible Monopoly set from Neiman-Marcus, an $8 inedible McDonald's soapburger - with a lettuce washcloth - from Camilier & Buckley, a $25 Unidentified Flying Bear in a mylar space suit from Kaliedescope, a $1.59 rubber duck that swims and quacks from Miles Kimball.

The four-color hucksters are marketing them all this year through the mailboxes of everybody who ever ordered anything by mail or who owns a credit card or even has a zip code with good demographics.

September is the mailing season for most of the estimated 8,000 companies in the mail order catalogue business. Not even the Catalogue Council of the Direct Mail Marketing Association knows how many catalogues will be mailed this year, but it's in the billions.

Sears, Roebuck & Co. prints 6 million copies of its big book, which really isn't a mail order catalogue since most of Sears catalogue business is handled through catalogue desks in stores. The big spring and fall catalogues are part of an extensive catalogue program that, with specialty catalogues - auto parts, office supplies, tools and the Christmas Wish Book - produces about 20 percent of Sears' $17-billion-a-year business.

For Sears, J.C. Penney and MOntgomery Ward, the catalogue is a way to stretch their inventories and broaden their selections. Catalogues allow the mail-order-merchants-turned-chains to offer items that don't sell very fast in any one store, but which add up to a million-dollar market on a national basis.

That's one way of doing business by catalogue. Another is the catalogue showrooms, like W. Bell, Best Products and Evans, which limit their merchandise selection to what's in the catalogue. Showrooms too do little of their business by mail and like Sears try not to mail the full-size catalogues because postage costs are too high. But teaser catalogs are now arriving in the mail of prospective customers, inviting them to pick up the regular book.

A recent innovation is for conventional department stores to publish a catalogue based largely on their regular merchandise. The appeal - as with all catalogue shopping - is convenience no parking hassles, no running around the store, just fill out the order or dial the number and the shopping is done. Hecht's, Woodward/Lothrop, Bloomingdales and I. Magnin all mailed fall catalogues to their Washington area customers in the past few days, as did Nelman-Marcus, which has developed the department store specialty catalogue into high art, high volume and high profits.

The N-M catalog this year is actually three catalogues. The teaser, called Christmas Previews 1978, went out last month. Behind its candy-stripped cover is a mixture of upper-middle-class baubles - ultrasuede tablecloths, monogramed tortoise combs with sterling initials, money clips and slippers for him, closonne and crepe de Chine for her.

The regular Neiman-Marcus catalogue - the one with the munchable Monopoly set - was hand-delivered to news media last week, and will be mailed to customers in time to capitalize on the publicity.

New this year is Neiman's Nelson Rockefeller Collection, thinner than the regular catalog, but with a cover the color of a young Bordeaux, enscribed in gilt. Georgetowners who were outraged by Rockefeller's decision to turn his estate into a housing complex undoubtedly will be shocked at the sacrilege of selling reproductions of the fabled Rockefeller art collections.

The rest of the world will be shocked only by the prices, which are upscale even by Neiman-Marcus standards. For the fireplace, bronze andirons designed by Giacometti in 1939 are $750. For the table, Meissen "Swan Service" china, an 18th Century pattern, $625 the place setting. For her, Mesopotamian earings, $395, pre-Columbian pins, $695; or assorted African artifacts, $95 to $4,250. For him, the bill, which will be big enough that there is no need to apologize that the gift is only a knock-off of the original.

The Rockefeller catalogue preserves the exclusivity of the Neiman-Marcus mail order operation, which has been flatteringly imitated by Houston retail rival Sakowitz, the Horchow Catalogue and, from Altanta, the Kaliedescope catalogue, a favorite of the Georgia crowd.

Despite the high prices that mean big average sales, the fanciest catalogues are plagued by what Washington mail marketing specialist John Jay Daly calls, "mail order tire kickers" who order the bait but have no intention of biting.

Daly said the return from a catalogue mailing - the number of customers who place an order - can vary from one percent to 6 or 7 percent, depending on the quality of the catalogue and the quality of the mailing list.

With catalogue printing costs running from a dime to a dollar or better, and postage costing 8.4 cents a piece or a minimum of 36 cents a pound for catalogues delivered to the post office in bags pre-sorted by zip code, increasing emphasis is being placed on the mailing lists.

Details of the financial performance of catalogue sellers are hard to come by because few are published by publicly owned companies. Gross margins are generally said to be higher for catalogue sellers than for other merchants, although catalogue showrooms almost universally offer discount pricing and as a result operate on margins in the 20 to 30 percent range.

Most mail order catalogues rarely claim lower prices even on items such as small appliances and consumer electronics that are widely discounted. Catalogue prices are hard to compare with retail stores' prices because many items are exclusives sold only through a particular catalogue. Some manufacturers and importers have separate lines for catalogue sellers, asking comparisons more complicated.

But comparison shopping is getting easier because, as the number of catalogues grows, the number of duplicate items increases. Experienced catalogue shoppers point out that even when the price of an item is two catalogues is identical, the actual cost may vary because of different mailins and handling charges.

With catalogue customer lists widely available, and increasing numbers of items turning up in more than one catalogue, the general merchandise catalogue business is becoming competitive to th point of saturation, some industry observers fear.