IT'S HARD TO know where to begin. There's so much intriguing material that destroys myths across the food spectrum and explains the previously inexplicable.

Are you surprised to find out that fertile eggs don't necssarily come from happier hens? That broccoli leaves are good to eat? That apple juice is a waste of money? That if you add a tablespoon of fat to each cup of dry beans as you cook them, they won't foam up?

Who says? The home-economists employed by the country's oldest food cooperative, the Berkley Co-op in California. And they've been offering these little gems to shoppers in their stores for the last 25 years, back when most people still thought that anything you could buy in the grocery store was good for you.

But the rest of the country hasn't come face to face with such practical hard-nosed advice in the form of shelf markers, in-store displays and weekly flyers . . . until now. The best of the advice has been put together by some of those same home economists in "The Berkeley Co-op Food Book." The authors don't play favorites: They are as critical of what big business sells as they are of the fringe. Natural vitamins and Campbell's soups both take their lumps.

The rest of the world may have caught up with the Berkeley Co-op, but there sstill aren't many supermarkets which are willing to tell you that some of the things it sells are not only bad for you, but crummy buys. Berkeley Co-op does. Some of its more militant members don't think it's critical enough. They would like to remove the junk foods from the shelves along with products that are "politically" inappropriate. Those are different things at different times. When porpoises were being slaughtered because of the method used for catching lightmeat tuna, the most militant wanted the tuna removed from the shelves. Sometimes the store responded to such a request. Today it is more likely to leave the product on the shelf and try to affect buying habits through an educaton campaign, just as it did so successfully several years ago with highly sugared cereals.

Warning shoppers about the hazards of these products, with advice on nutritious, reasonable alternatives resulted in a 50 percent drop in sales of the highly sugared cereals.

One article is entitled "The worst bargains in the supermarket." It's short, like all the articles, and to the point. The honor roll includes bacon, hot dogs and spareribs because . . . you get less meat, more fat or bone than in most other cuts.

"The 3 1/4 ounce size Chicken of the Sea tuna because "the price per can may be low but the price per pound is the highest of any size can tuna."

Hamburger Helper because "you're paying a lot for a pinch of herbs, some dried tomato sauce, about 1 1/2 cups of macaroni! And you stil have to buy the meat."

Del Monte individual pudding cups: because "that's a lot of packaging for a little bit to eat . . ."

Birdseye frozen vegetable in sauces because "with all that sauce there' less vegetable. That means less of the nutrients which are hard to get, and more nutrients (fat and calories!) which are too easy to get!"

That straightforward advice is found in a chapter entitled "Before You Shop." It is preceded by one on how to be an active, informed consumer and followed by two on food buying and food safety. While some of the articles are six years old, they are as pertinent today as they were when they were written.

In 1974 Helen Black, who edited the book, wrote about UPC, the universal product codes on products which make it possible for an electronic scanner to read the prices at the checkout counter without the price actually being on the product. That makes it possible for a store to remove prices from individual items and just use shelf markers, something stores discussed doing serveral years ago and Giant tried in this area. The stores backed off when shoppers complained it would be hard to compare prices or make sure they weren't being shortchanged at the checkout counter. But now the stores are back trying again in several states and Giant announced last month that it would not individually price mark in its new test store. At the store opening, Giant's president, Israel Cohen, said individual price marking adds nothing to the intrinsic value of the item.

"There is no benefit to item pricing," Cohen said.

Black wrote: "Already I'm getting nervous. Already it's very hard to compare prices of foods. Without a price on the item, even if it's well-marked on the shelf, it'll be even harder. What if I want to compare the price on canned kidney beans, stocked in another section?

"Supermarkets have been known to be a little sloppy about shelf prices . . . Most frightening is the ease with whcih prices can be changed in a computer. How can shelf pricing keep up?

"At the checkstand . . . if a mistake is suspected, someone will have to go back to check the shelf price to be sure."

Six years later Helen Black's concerns are right on target.

In the practical advice department the book succinctly details the differences between nectar, punch, drink, ade, cocktail (then it tells you how to make your own juice drinks at home); it describes the differences between carob and cocoa and concludes that "carob used in candies in place of chocolate or cocoa has no nutritional advantage . . . ; it even lists beans in order of gasiness (soybeans are the most; black eyed peas are the least).

More space is devoted to the subject of cereals than almost any other topic.

Co-op's attitude toward highly sugared cereals is summed up in a quote from Dr. Joan Gussow, chairman of the program in nutrition education at Columbia University Teachers College: "The question is whether it is okay to enrich garbage. We simple don't know enough to keep someone alive on enriched garbage."

But instead of just railing against the evils of highly sugared cereals the book offers lots of good advice on alternatives.

There's an excellent quiz on the differences between various milk products which produces some information that may surprise health food fanatics. Some people think that non-instant dry milk is processed at a lower heat and is therefore more nutritious. No says the Co-op. "The process used to instantize nonfat dry milk does not involve heat at all."

The vegetable chapter is filled with little-known facts of interest and use.

There is a list of the nutrient value of vegetables which categorizes them -- the best, second best, still excellent, very good and good nutrients. Collard greens and corn are the only ones in the best category. There are instructions for cooking frozen vegetables which differ significantly from those found on the packages. "For each cup of frozen vegetables put in pan and bring to boil 1/2 to 1 tablespoon water . . . " If you want to remove the wax coating from vegetables soak them for two or three minutes in warm dishwashing detergent suds and then wipe with a towel and follow with thorough rinsing. When a store sprinkles leafy vegetables it's good for the vegetables: rIt helps prevent wilting which causes loss of vitamin A and C.

The book dispels the notion that canning vegetables kills off vitamins. It also dispels the notion that Campbell's condensed soups are nutritious but says that the chunky varieties aren't bad.

Obviously some people thing Co-op's advice is pretty radical but its actually pretty middle-of-the-road and can be summed up in a warning they issue on the subject of improving family nutrition: "Do not expect your family to eat foods just because they are 'good for you.'"

The book is available by mail for $8.50, including postage. Write to: Berkely Co-op, 4805 Central Ave., Richmond, Calif. 94804.