DECORATING BOOKS generally are most useful when serving to correct shimmying table legs. They can also be used as a hot pad for the teakettle, a press for linen napkins and even, in a pinch, as fillers for bookshelves until you complete your set of encyclopedias-by-the-month.

A recent exception to that rule is "Decorating: A Realistic Guide," by Mary Gilliatt (Pantheon Books, $25), a book actually useful for the sensible ideas it proposes, and all with high good humor. Unlike what sometimes seems like 99.5 percent of the writers on design, Gilliatt is both a good writer and a good designer. One precaution: Read the book when you have time to leap from your chair and start pushing furniture around. You'll find it affects you that way.

The book has extraordinarily handsome color photographs - 280, according to the publisher. The photographs in the book were taken to illustrate the text rather than, as is too often true, being gathered from handouts from linoleum companies.

You won't find anything ticky-tacky in this book. The accent is toward mid-century modern, true, but it gives room to neat old places. Gilliatt, an author of house books and a general assignment writer on British newspapers and magazines for years, is now design and furnishing coordinator for Liberty's of London as well as a free-lance interior designer.

In town a while back to promote her book, she admitted that as a writer, she's sick and tired of model rooms where "the designer had no idea in the world of the price. When I've done model rooms for Liberty, I tried to make them as practical as possible - six rooms each costing under $200. There really is no point in $20,000 mirrors. You have to keep a sense of proportion . . . I believe in simple ideas . . . the stapled Indian bed-spread on the walls, for instance."

On the other hand, Gilliatt thinks that people today have "rediscovered quality. We can't afford the throwaway mentality any more. We're giving up on fads. I thought 10 years ago that we were in for radical changes in design. But not now. Now we're back to nostalgia, quality and the beautiful. It's a more eclectic way of life, and everyone has to take us as we come."

Gilliatt reports that British designers are, like Americans, much interested in producing objects themselves. "Royal College of Art now graduates 25 percent of its people to industry, 75 percent to the crafts."

In Gilliatt's townhouse in London, according to Christian Science reporter Marilyn Hoffman, the colors are apricot, pink, brown and green in all the rather small and narrow rooms. Gilliatt has placed a desk of glass and lucite and plastic cubes to heighten the sense of space.Extra seating is provided by a window seat. And Roman blinds keep the windows bright and uncluttered.

Gilliatt is very much preoccupied with light. And she knows how to use it. For the cover of her book, Gilliatt and her editor, Barbara Plumb, tried to produce with photographic lights (instead of chandeliers and a lamp) just the sort of light and color one sees in the best of the Flemish old master paintings. A splendid black ladder, a soft chair, two lengths of fabric and a bunch of flowers complete the spare but warm effect. What the cover does manage to show is how beautiful a room can be with hardly any "decorating" at all.

Those of us who have moved often can attest, to our shame, that most often a room or apartment or house looks better before we put anything in it. Therein lies a rule that most of us never learn: Space - plain, uncluttered, undecorated, unsullied - is the most luxurious of all components.

Gilliatt points out in the beginning of her book that the photographs in the book are not only of the houses of professional designers, but also those of "painters, illustrators, writers, architects, landscape designers, manufacturers, academics, teachers, dress designers and just generally creative people with an eye for color and detail."

After bolstering your confidence thusly, she goes on to offer practical suggestions. Though her advice seems very useful, and indeed on some points brilliant, probably the chief contribution of the book is bolstering your confidence in solving your housing problems in your own way, no matter how strange or how ordinary your idea is.

What follows might be thought of as fabric swatches or paint chip samplings from her book.

"If the thought of discomfort ahead is a melancholy one, remember that if the place has to be lived in while all this is going on - and most do - it will help to keep spirits up if at least one and preferably two rooms can be made comfortable right away, if by no means in their final form.

"In a kitchen-living room, this can be achieved with surprising ease, and without benefit of paint or paper if need be - though these certainly help - by adding a cheerful tablecloths, plants, prints or posters, good-looking pillows, a rug or two and any decent pieces of furniture that are already in existence.

"In a bedroom, it helps to have a comfortable bed, a rug, a good, well-lit mirror, a reasonable amount of storage space and a chair. Or you may have the bed in the living room and cover it with pillows or cushions so that it can be used as a general lounging area. Bed manufacturers would disapprove, but never mind. Some friends of mine got so used to having their beds in the so-called salon that they never ded get around to having a proper bedroom and turned the space into a library instead."

To save money, she suggests hunting for architectural oddments in junkyards and carpet ends in remnant stores (watches out for fire-or flood-damaged goods - they rod. She's in favor of wood floors, painted or polished, and homemade window shades from fabric remnants. One of the more remarkable ideas pictured (yet it could certainly work) is of truck tires slipcovered to serve as seating in what appears to be an otherwise unremarkable room, lighted with photo flood lamps.

To expand space where you can only rearrange the furniture, not the walls, Gilliatt suggests: "Two small sofas look neater than four chairs. Two small seatings units pushed together take up less room than two individual chairs spaced apart. Storage all down one wall is better than separate desks, liquor cabinets or trays. . . . Fold-up and thus get-riddable-of furniture should be used wherever possible."

She also suggests that "a mirror on a table or mantelpiece with a plant or small objects in front of it will give depth, as will a hinged screen with, say, a small table and lamp in the foreground. Similarly, the eye can be drawn out and along by diagonal or zigzag lines painted on doors or walls, or by similar geometric or directional patterns on the floor."

One of the favorite architectural tricks of today, the multi-level room, is shown here in the home of Michael Dunne, who took most of the photographs in the book. Dunne, his wife and five children fit into a mews house - about like a Washington carriage house - with the help of many levels and a spiral staircase.

The photographs of various forms of lighting are especially dramatic - Paul Rudolph's apartment, for instance, with strips of light on the floor, that turn the night view of New York into wide highway of light. Gilliatt recommends using wallwasher lamps, or lights set in plug-mold to enlarge space; low lamps to lower a ceiling or make a room seem more intimate; uplights in a corner to define the limits of a room, set under glass shelves for sparkle or behind plants for interesting shadow.

After dealing with storage (clothes can be hung from pipes in the ceiling if necessary), color (white is always safe), texture (she's in favor) and ways to keep everything from being too serious (glasses on the marble bust of Caesar), she relates the tale of the woman notorious for driving decorators mad. The woman framed and hung in her visitor's bath a letter from the last of her decorators:

". . . Now that we have exhausted the possibilities of the whole United States, Great Britain, Italy, France, Switzerland, Scandinavia, India, Thailand, Hong Kong, etc., etc., and still not found anything to suit you, I feel that I can have little else to offer you in the way of ideas and therefore resign my commitment to you. . . ."