BED & Breakfast, a people-to-people response to the high cost of tourist accommodations, continues to spread across the United States with the fervor of a revival meeting.

For the true believer, no sterile "assembly line" hotel-motel room can offer the warmth and cozy comfort found when sharing someone's home -- even though some rather expensive B&Bs have become more like class acts than low-cost, folksy havens; and many new or renovated hotel properties are improving their image by emphasizing non-plastic, imaginative design features and more personalized service.

Still, the B&B price is usually right (if no longer a wild bargain) when compared to $100-a-night-and-up hotel rates. Thus the troubled U.S. economic picture probably constitutes the most important factor behind the current boom. That's why more couples, widows, singles and divorced Americans, from blue-collar workers to professionals, are seeking to earn a few extra dollars -- while making new acquaintances -- by opening up their spare rooms to transient visitors; and why more tourists are willing to spend a night with strangers.

And, fortunately, for those who are concerned about just which stranger they might want to face across what breakfast table, a spate of new guidebooks and other publications are now beginning to help perplexed, dollar-conscious travelers with their oft-heard plaints: "How can I find a B&B in . . . ?"

Aside from the low cost, guests often report additional reasons for the growing popularity of the Bed & Breakfast alternative.

These people were very gracious and interesting to be with. Hosts really went out of their way to show me Billings and environs. We had a trip to Yellowstone and Custer's Battlefield which was memorable and fun.

For two tired, time-disoriented people returning from China, to come to that totally light-proofed little guest house in Carmel with the sherry and nicely stocked refrigerator, it was a real welcome. Members of The Bed & Breakfast League

The B&B concept was born abroad, in the British Isles and elsewhere in Europe, though of course Americans throughout their history have always operated guest houses (sometimes called "tourist homes") or informally taken in boarders. The B&B, then, differs in its purest form from the professionally managed commercial hotel, or even the standard inn, in that one or more rooms in a private home are set aside for travelers passing through town -- generally for only one or two nights' lodging. The owners of the house provide a clean bed and bath (not always private), usually share their living room, and offer the visitor congenial company and helpful information about the area in which they live, plus often a home-cooked breakfast at the family table.

However, the growing demand for budget accommodations across the country, and the proliferation of B&Bs, has resulted in a confused terminology. Some guidebooks designate as a B&B anything from a ranch or farm to a lodge, apartment, guest house, tourist home or country inn. In addition, families will discover that children are not always welcome (pets rarely), and even that breakfast may be either unavailable, reduced to the continental variety, or served only for an extra fee.

The facilities may be modest or luxurious, the structure modern or historic. According to one registry service's simple definition, "A B&B is an accommodation in a private house or small inn where the traveler is treated more like a guest than a source of income."

Not too long ago, a standard B&B in England could be found for only about $10 a night per person, but inflationary pressures have caused many prices to double except in the small villages. In this country, too, rates have been rising coast to coast due to increased operating expenses.

As always, leave it to a California agency to add some special touches to the basic B&B concept. California Houseguests International, 6051 Lindley Ave. No. 6, Tarzana, Calif. 91356, (213-344-7878), includes in its listings "an oceanfront mansion with a huge deck overlooking 60 feet of private beach in Malibu complete with movie-star neighbors, live-in maid, piano . . ." which rents for $60 a night double. Or maybe you prefer for the same price "a grand Spanish hacienda, high over the Pacific, decorated with ethnic antiques, owned by a professional couple, gourmet cooks who offer meals as an extra option. The beach 40 ft. below can be reached by a tunnel . . ." Typical B&B double rates "in the heart of Los Angeles" are $45 up.

There are B&Bs for homosexuals (Bed-By-The-Bay, P.O. Box 902, Sausalito, Calif. 94966, calls itself the nation's "first and only gay bed-and-breakfast agency").

There are B&Bs for educators, active or retired (Educators' Vacation Alternatives, 317 Piedmont Rd., Santa Barbara, Calif. 93105, sells a combined home exchange and B&B directory which accepts new listings -- for an additional fee -- and describes properties, giving addresses and rates; guests book directly with the hosts). When ordering a directory ($5.50), you must include a statement identifying your past or present educational employment.

Some hosts become unofficial concierges, suggesting places to go and see, arranging tickets and acting as guides. Some offer dinner as well as breakfast . . . Many of the guests . . . have literally become members of the family . . . returning year after year. Bed & Breakfast Colorado

Then there's the business outlook for those who are thinking of playing hosts. In her preface to "Bed & Breakfast U.S.A.," Betty Rundback notes certain tax advantages to operating a B&B in an "otherwise-too-large-house," and points out that "it is a unique business because you set the time of visit and length of stay . . . and the extras, such as meals, are entirely up to you."

But Norman T. Simpson cautions in his "Bed & Breakfast American Style," that successful B&Bs often depend upon location, and suggests potential hosts ask themselves questions such as, " . . . what are your feelings about having people using your home and your belongings; are you prepared to give up privacy . . . ?"

While in Great Britain and Ireland it is quite common to find public displays of B&B signs, they are unusual in the United States. There is an obvious hesitancy among many American homeowners to throw open their doors to strangers without some sense of control. They choose instead to use an agency/reservation service approach, which limits direct access by potential users to property owners' names and addresses.

Travel agents, too, have begun to act as intermediaries. P.T. International, a reservations service based in Portland, Ore., that deals only with the travel industry, has compiled a list of 15,000 rooms worldwide, including about 30 states, Puerto Rico, the U.S. Virgin Islands, Canada, Great Britain and France.

Rates with continental breakfast at properties in Portland, for example, start at $33 a day double, while in San Francisco they range from $38 to $200, according to vice president Bill McAlpine. When a tourist asks a travel agent for a B&B at a specific destination, the agent merely calls P.T. on a toll-free number and requests the booking. When confirmation is received, "in from 12 to 24 hours," McAlpine says, the client pays the full amount in advance to the agent, who issues a pre-paid voucher and deducts his commission.

P.T. International has representatives in every area where their B&Bs are located to inspect the properties, according to McAlpine, and the firm stands behind the descriptions given to agents. He explained that for a $33 rental fee, the travel agent gets 10 percent, the rep who inspects is paid $3, the homeowner earns $20, and the firm retains the balance -- $6.70.

A few current B&B guides (not the five books reviewed here) consist merely of loose, mimeographed pages listing B&Bs in many states, while other compilations are more professionally prepared in booklet form and may be limited to one area. They are usually priced at $2 to $4. Most listings are of B&B agencies, rather than giving names and addresses of individual properties and their owners. These agencies, run as informal businesses by people who enjoy providing this kind of service, have a central reservations "office" with phone numbers for booking guests.

Some agencies sell their own directories, which show U.S. and Canadian cities where affiliated B&B hosts they represent are located and describe the properties (without revealing addresses) in terms of proximity to tourist attractions, number of rooms, rental charge, special rules, etc.; for an additional fee they will list your property in these publications. Others provide free directories. In some cases there is also a membership fee.

The Bed & Breakfast League Ltd., at 2855 29th St. NW, Washington, D.C. 20008, was the first U.S. membership group (formed in 1979 in Princeton, N.J.) and one of the first agencies. Directed by Diana MacLeish, the league now has hosts in 35 states, Canada, the U.S. Virgin Islands and England. Members and hosts pay an annual fee. Twice a year the league publishes a directory of its hosts that describes each property but gives no addresses, and this is mailed to members who also receive a toll-free phone number for reservations.

Single rates range from $25 to $32, doubles $28 to $38, with at least a continental breakfast. Full payment must be made in advance -- two weeks is advised -- to the league (personal check, Visa, MasterCard), which then confirms the booking, deducts 15 percent and pays the balance to the host. It will also refer members to other agencies when necessary. The office is open from 9 a.m. to 5 p.m. weekdays (232-8718).

The league is offering a free, four-page list of B&B agencies around the country, and their own brochure. To receive one, send them a large, self-addressed, stamped envelope.

Another local agency that has prepared a list ($2 charge, five pages) of B&Bs, and guidebooks, along with details on its own operations in the District and Annapolis, Md., is Sweet Dreams and Toast Inc., P.O. Box 4835-0035, Washington, D.C. 20008 (483-9191).

Also operating in Annapolis, and on the Eastern Shore and in Baltimore, is a new agency, Sharp-Adams Inc., 8 Gentry Court, Annapolis 21403 (301-269-6232). They handle homes, historic inns and "stationary yachts," says Cecily Sharp-Whitehill who works with partner B.J. Adams. Their first directory is not yet completed, but the top rate is currently around $50 -- and that's what you'd pay for a night aboard a yacht docked on the Chesapeake Bay (Sharp-Adams charges $5 a night per room reservation, then guests pay rental fees directly to the owner).

Some agencies indicate an effort is made to screen either the B&B property, the user, or both, through use of a questionnaire, phone interview, request for references or combination of these methods, and -- in certain cases -- actual inspection of the property. In many instances, though, it is a matter of mutual trust.

In Charlottesville, Va., the six-year-old Guesthouse Reservation Service handles deluxe B&Bs charging from $48 to $90, plus tax. Sally Reger makes a special effort to match guests with hosts who are "proud of their homes and eager to share them" but who depend upon Reger to send them compatible visitors. All homes (and cottages) undergo "rigid inspection," Reger says, and she frankly emphasizes "quality" over economy. The agency's address is PO Box 5737, Charlottesville, Va. 22903 (804-979-7264 or 979-8327, 1-6 p.m. weekdays).