In an era when the boldness and simplicity have become hallmarks of French cuisine at its best, it is appropriate tht an important new book for those who would cook in the French fashion has been produced from a bold and simple concept.

The book is "La Technique" (Quadrangle, $20) by Jacques Pepin, a gifted chef who has worked and taught cooking in this country for nearly 20 years.

The subtitle, "an illustrated guide to the fundamental techniques of cooking," explains the concept. Through photographs Pepin shows the reader the step-by-step methods of creating dishes across the panorama of the French menu. It's television's Julia Child in still life.

Pepin believes, along with most serious teachers of cooking, that the heights of French cuisine are scaled through arranging and rearranging a series of building blocks. These blocks are teh fundamental techniques of the most structured of all national cuisines. Once learned, the cook is free of a slavish devotion to recipes. Ingredients change but the techniques don't. It becomes second nature to convert stocks into soups or sauces. Roasting, poaching, sauteeing are clearly defined, awaiting only the appropriate cuts or types of meat or fish. In short, knowing the beginning, middle and end of the recipe plot, one needn't become lost in mundane detail or scared of complexity. The cook can concentrate on perfecting execution. He or she becomes free to improvise.

Traditionally, this is done by working beside a more experienced cook, learning basics by observation and trial and error, all the while absorbing "tips." Pepin cannot save the reader from the anguish of error. In cooking you have to feel what is right as well as see it. What he can do, and what recipe books unfailingly fail to do, is illustrate the way-steps to perfect execution.

He begins by showing how to hold a knife and use it. As the book progresses through 170 techniques and 450 pages, he reveals methods of making pastry, of cooking and cutting meats and fish, of making exotic garnitures such as tomato roses and mushroom medallions or something as basic as separating, peeling and chopping garlic.

There are recipes, lots of them. Make stuffed eggs or crepes Suzettes. Make melba toast. They are straight-forward, though, because they are teaching tools. Follow them and the food will be good, but there's no cuisine minceur or shortcuts with convenience foods. There's no razzle-dazzle with herbs, spices or condiments, either. That's for you to add once you have a absorbed the techniques.

The photographs are close-ups and reasonably detailed. Naturally, they lack a third dimension and are therefore imperfect. But they have been well-planned and executed. You do see what you should see. There are as well eight pages of colored plates in the heart of the book. Probably intended to relieve an inevitable textbook appearance, they reveal instead a compromise with quality on the part of the publisher.

This need not concern potential purchasers. Considering the number of unused recipe books home cooks surround themselves with, considering the going price of cooking lessons, this book represents a bargain for anyone who uses it in the spirit of its author's intention. Pepin would not argue, I think, that a chef can be a genius. But he has clearly demostrated for those outside the profession that the chef is not a magician.