The following B&B guidebooks, all paperbacks, differ in style and approach, but each is valuable. They prove it is still possible in some places to get a double with full breakfast for under $30 a night, though all authors caution that rates are subject to change.

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

"The Great American Guest House Book," by John Thaxton, illus., 261 pp., Burt Franklin & Co., $7.95.

These two guides list many B&B agencies, which are reservation services (the first guide gives more detail). They also include many individual properties and addresses (the second book is more descriptive), and give rates. They often duplicate each other's coverage and sometimes differ as to specific facts about identical inns -- which may result from changes by owners and does not lessen the value of either book. Rundback covers 47 states and Canada, Thaxton 38 states and Canada.

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

Simpson is the author of the well-known, up-scale "Country Inns and Back Roads," now in its 17th year of publication. He has borrowed some of those listings for this book, but has also added many new properties whose rates range from $15 (without breakfast) to $140 (three meals).

His listing covers 29 states and Canada.

Simpson notes: "Inclusion in this book is not based on budget travel Simpson has never focussed primarily on price , though some places have rates that compare favorably with nearby commercial facilities." He has chosen a variety of properties -- "impressive urban mansions, many on the National Register of Historic Places . . . trim farmhouses . . . comfortable country inns . . ."

"The New England Guest House Book," by Corinne Madden Ross, illus., 209 pp., East Woods Press, $7.95.

This guide describes 186 guest houses in six states. Though specific rates are not given, each property is classified as either "inexpensive ($10-$25), "moderate" ($25-$45), or "expensive" ($45-up).

"Victorian Holidays," by Marsha Cudworth and Howard Michaels, illus., 94 pp., $5.95 plus $1 postage, from Bric-A-Brac Bookworks, P.O. Box 887, Forked River, N.J. 08731.

A readable, authoritative look at five "guesthouses," 10 "bed & breakfast inns," and six restaurants in Cape May, N.J., "America's Oldest Seashore Resort." Cudworth and Michaels blend history, good illustrations, useful information and an obvious love of their subject so that prospective guests should have a clear idea of what's in store for them.

Note the warning the authors give about weekend vacation reservations being "an absolute necessity. Most inns have under 10 rooms each and many have established clientele who have booked and paid for rooms in advance."

Travel guides that cast a critical eye on their subject fulfill an important need. In the case of these five guides you probably will not find a word of critical comment -- only warmly-approving descriptions -- but there's a good reason. Simpson and Cudworth/Michaels were able to include solely those B&Bs which met their standards after a personal visit; in other cases, all properties had to at least provide answers to specific queries by the authors if no inspection could be made, and many received recommendations from others who did visit. All the authors are asking for feedback from readers who stay at these B&Bs.