Some years ago this September, I came up against an exceptionally fertile plum tree. Its yield surpassed the greed of my plum-loving friends; jam jammed my shelves, and still the tree bore fruit.

I finally brought it under control with a recipe for plum catsup. That, I think, is when I went astray.

Till then I had lived in the belief that catsup grew in bottles, peanut butter was harvested by a patient people who didn't mind getting their fingers sticky and, if you went to California, you must be sure to see the raisin trees.

Like all converts, I went too far: I blended my own peanut butter; Rendered lard; with neither pasta machine nor an Italian grandmother to hlep, I rolled out the heaviest pasta ever to sit under tomato sauce; I pressed on friends my recipe for soybean loaf.

My advance into the past ended with 20 quarts of soggy pickles. Suspicious of one of the ingredients, I had left out the thing that gives the little fellows their crunch.

All of this is because I've come across a book I wish I had had then: "Better Than Store Bought" (Harper & Row, $12.95). Its authors, Helen Witty and Elizabeth Schneider Colchie, have revealed the mysterious ingredients of such supermarket staples as liverwurst, pickled herring and hamburger buns.

Even better, they give recipes for things you might not be able to find. If you're nowhere near a Spanish store, but still want chorizo, they tell you how to make it. Would you like to make Gravad Lax or sauerkraut? It's all here, along with recipes for preserves, chutneys, candies and even, heaven help us, fig newtons.

There are also sensible instructions on how to save the things you have-- herbs, for instance, and which to dry, which to freeze, and which to steep in vinegar-- or how to crack a coconut or make a confit d'Oie. The latter, like the liverwurst, may take a strong commitment to cooking. But other things are simple. Recipes for two of the easier items, olives with bay leaves and rosemary, to be served with drinks, and crystalized mint leaves, to serve with after dinner-coffee. Olives with Bay Leaves and Rosemary 2 cups (about 10 ounces) small green olives with pits, preferably cracked 2 bay leaves 1 tablespoon dried rosemary 1/2 lemon cut lengthwise, then crosswise into thin slices 1 teaspoon whole black peppercorns, brusied Olive oil to cover 2 or 3 cloves garlic, peeled but left whole

1. If necessary, crack the olives as described above. Rinse the olives and layer them in a clean jar with the bay leaves, rosemary, lemon slices, and peppercorns. Pour in enough good olive oil to barely cover the olives (they will sink a bit). Add the garlic and cover the jar.

2. Let the olives stand at room temperature for at least two weeks, stiring or shaking well daily, before serving them. They will keep for months, preferably refrigerated. Allow to reach room temperature before serving. Crystalized Mint Leaves Freshly picked mint leaves 1 egg white, at room temperature Few drops of water, if needed Extra-fine granulated sugar

1. Pull or clip each mint leaf from the stalk, leaving, if possible, a short stem on the leaf.

2. With a fork, beat the egg whites in a saucer until it is spreadable (a few drops of water may be added if the white seems very viscous). Have at hand a plate on which to "paint" the leaves, a plate of extra-fine sugar and a cake rack, covered with waxed paper.

3. Using a small paintbrush, cover every surface of a leaf with egg white in as thin a layer as possible. (Tweezers will help in holding the leaf.) As you coat each leaf, lay it on the sugar and sprinkle more sugar over it until it is evenly coated. Lift it to the cake rack and continue until all the leaves are coated.

4. Set the rack of leaves in a warm, dry place, such as an oven heated by a pilot light. When the leaves are superficially dry (they'll still be moist inside), transfer them to an uncovered rack and continue to dry them, if necessary, for several days, until no moisture shows when a crisp leaf is broken open. Store in layers, separated with waxed paper, in a cardboard box. Kept dry, the leaves will keep for many months.