BED & Breakfast accommodations continue to thrive in the United States and new guidebooks are being published while others are updated, but many potential users remain unsure about how B&Bs function--and how to go about finding one along their route or at their final destination. There are still a number of misconceptions.

B&Bs are designed to attract travelers who are happy to share someone's home, especially when the temporary "togetherness" can cut the cost of their overnight vacation stays. While it is true that American economic troubles and rising hotel/motel costs have been major factors behind the B&B boom, many travelers also welcome the change from impersonal commercial establishments.

Thousands of B&Bs are now being offered in at least 1,500 cities in this country, Canada, Mexico and the Caribbean. More than 150 organizations are handling guest reservations. But there is currently no all-inclusive, continually updated and easily accessible listing. The B&Bs are operated by no one special group--couples, widows, divorced and single homeowners, condominium and apartment dwellers, farmers, ranchers. These outgoing hosts, who are seeking to earn a few extra dollars while making new acquaintances, set aside one or more rooms for tourists passing through town--generally for a few nights' lodging.

They agree to provide a clean bed and a 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. Facilities may be modest or luxurious, modern or historic.

If you're planning to try a B&B, here are some tips and guidelines:

* Reservations are essential and usually must be made two or three weeks in advance. Some B&Bs require minimum stays; some do not permit children, pets or smoking.

"We have not yet begun to address the question of making B&B readily available for those who are unable to plan ahead and must delay arranging lodgings until after they have arrived at their destination cities," notes Patricia P. Wilson, founder and president of the Washington-based American Bed & Breakfast Association, whose members are B&B organizations. "At present there are few organizations that can handle last-minute requests," Wilson says.

* Many North American B&B hosts are reluctant to throw open their doors to strangers without some sense of control, so they use an agency/reservation service approach. This limits direct access by potential users to property owners' names and addresses.

Guidebooks, and compilations provided by some reservation services and others, list basic information about various properties (general type of property, facilities, restrictions, rates). If addresses and phone numbers of individual hosts are not given, interested guests must contact the respective reservation agencies as listed. The agencies handle bookings for a commission, and some check out host homes before agreeing to represent them. They may also attempt to screen guests through use of a questionnaire.

* In some cities, the local chamber of commerce will be able to provide assistance in locating B&Bs. Soon the Yellow Pages of U.S. telephone directories will begin carrying a "Bed & Breakfast Accommodations" heading, under which can be listed individual properties and agencies in their areas. (Editions of the Washington, Maryland and Virginia phone books will soon include that classification, according to a spokesman for the C&P Telephone Co. The first local directory to list B&Bs will be the Maryland suburban issue for 1983, scheduled for delivery in about a month.)

* If you prefer to have your travel agent book you, ask him to contact P.T. International, a reservation service based in Portland, Ore., that deals primarily with the travel industry (and pays a commission) but also handles direct bookings from individuals. It represents more than 20,000 rooms worldwide, including hosts and small inns in 48 states, Australia, New Zealand, France, Britain, western Canada, Mexico and the Caribbean.

P.T. covers more than 90 cities in California, which an official terms "the hotbed of B&B activities . . . In San Francisco alone we have between 400 and 500 rooms." Reservations can be made by calling a toll-free number, 800-547-1463 (prepayment is required), or you can request a brochure explaining the company's function and giving a list of cities served.

Although B&Bs "generally cost somewhat less than commercial lodgings," Wilson says, "it would be a mistake to go the B&B route for reasons of economy alone . . . B&B isn't for everyone" and it is not in competition with the hotel industry. While Wilson emphasizes that "nothing can match the special type of hospitality one finds in being taken into a host's home as a welcome stranger," she adds that "not everyone wants that kind of hospitality when they're traveling." Many prefer the convenience and privacy provided by hotels.

If you are thinking of turning that spare room into a B&B, Wilson has another pertinent bit of information. A recent American Bed & Breakfast Association survey showed that "less than 5 percent of all hosts earn any significant income" from their venture, with most reporting less than $500 a year. The most important reason they welcome the B&B traveler "is because they themselves enjoy people."

The Bed & Breakfast League Ltd., 2855 29th St. NW, is a special example of reservation agencies. One of the first to begin operating in this country (1979), it is a membership group that now represents 70-80 hosts in the Washington area, according to its director, Diana MacLeish. Included are B&Bs in the Virginia and Maryland suburbs and Annapolis. To book a B&B in those nearby areas, it is not necessary to become a member of the league, though the annual membership fee is required in order to use its list of 550 hosts in 45 states and Britain. Members receive a directory giving descriptions but no addresses.

MacLeish says the league has checked out all its homes in this area and attempts to inspect all other properties it handles, either personally or with questionnaires, photos and help from its "experienced guests." She points out that since members have access to a toll-free reservation number, the league can often make bookings on very short notice and accept payment with a VISA card or MasterCard. Rates range from $25 to $50 single and $38 to $70 double, with at least a continental breakfast.

A six-page list of U.S. reservation services and publications is offered for $2 by one local agency, Sweet Dreams and Toast Inc., P.O. Box 4835-0035, Washington, D.C. 20008 (483-9191). Sweet Dreams represents 90-100 homes located primarily in the District. Other Washington area B&B reservation agencies include Bed 'n' Breakfast Ltd. of Washington, P.O. Box 12011, Washington, D.C. 20005 (328-3510); Princely/Bed & Breakfast Ltd., 819 Prince St., Alexandria, Va. 22314 (683-2159); and Sharp-Adams Inc., 8 Gentry Ct., Annapolis, Md. 21403 (301-269-6232). They also specialize in nearby areas.

The American Bed & Breakfast Association, P.O. Box 23294, Washington, D.C. 20024, sells a "Hostlist" covering 5,000 hosts in nearly 1,500 cities. Priced at $5, it includes individual hosts and agencies. The fall/winter edition will be available by mail in mid-October. The AB&BA is a trade association and not a reservation agency.

Following are some of the most useful B&B guidebooks now available at bookstores or from the publishers (there are other regional guides, some self-published, and compilations of lists--photocopy products of varying value--are being prepared and sold by mail):

* "Bed and Breakfast in the Northeast," subtitled, "From Maine to Washington, D.C.," by Bernice Chesler, with maps, 509 pp., just published by The Globe Pequot Press, $9.95.

* "Bed & Breakfast U.S.A.," subtitled, "A Guide to Tourist Homes & Guest Houses," by Betty Rundback and Nancy Ackerman, illustrated, 212 pp., E.P. Dutton Inc., $5.95.

* "The Great American Guest House Book," by John Thaxton, illustrated, 325 pp., Burt Franklin & Co., $7.95; revised this year.

* "Bed & Breakfast American Style," by Norman T. Simpson, 292 pp., the Berkshire Traveller Press, $7.95.

* "The New England Guest House Book," by Corinne Madden, illustrated, 209 pp., East Winds Press, $7.95.

* "Victorian Holidays,"(on Cape May, N.J.) by Marsha Cudworth and Howard Michaels, illustrated, 104 pp., $6.95 plus $1 postage, from Bric-A-Brac Bookworks, P.O. Box 887, Forked River, N.J. 08731; revised this year.