It was inevitable that someone would finally spill the beans about middle age: that -- for women, anyway -- it can be the best age of all. That's the secret Alice Lake is telling in "Our Own Years." She means 35 to 55, when the children are out from under (to the relief of most women but the juices are still running. At mid-life, says Lake, many women for the first time can do something for themselves and often catch a second wind of energy, confidence and ambition. It is a time, she says, when women "finally grow up," become more assertive, less self-critical, more in control of their lives (with some exceptions: Lake tells about one husband who wrote the dean of the college his wife attended "commending him on her progress, but adding that she would not be returning: 'She hasn't kept up with sewing buttons on my shirts, and I think we'll keep her home for a while'").

On the whole, however, Lake sees middle age as a bonanza. To make the most of it, she has come up with a handbook of helpful tips. Her basic message is: Keep in Motion. When she says, "Use it or lose it," she's refering to muscles, brains and sex. The good news here is that sex is the fountain of youth and exercise "the nearest thing around to an anti-aging pill." The bad news is the obstacle course of health hazards to be run. If high blood pressure doesn't get you, diabetes may. Lake's catalogue of perils would wipe you out if it weren't for her cheerful approach and dozens of concrete suggestions about how to sidestep the worst. She hands out nononsense advice on estrogen, the pill, pregnancy after 35 -- in fact, everything from how to avoid Dowager's Hump to what to do if you think, God forbid, you have gonorrhea.

Lake includes some prescriptions for making it through middle age that I may embrodier on my sofa pillows:

Grouchiness is a survival technique.

My memory isn't failing -- it's just that my mental storage closet is too crowded.

I'm not growing old gracefully, but candidly, open-mindedly and flexibly.

Middle years are the best years of my life.

You'd better remember those upbeat words when you tackle "Women of a Certain Age," in which Lillian Rubin studies the mid-life transition of women between 35 and 54 who "gave up whatever jobs or careers they may have had in their youth to devote themselves to full-time mothering and housewifery." Most of them find out in middle age what a gyp life can be. Far from feeling grown up at last, Rubin's typical 45-year-old wife and mother "in important ways . . . remains like a dependent child -- unable to make her way alone in the world, wholly or largely dependent upon her husband for the most basic of life's necessities."

On some points Rubin and Lake agree: The "empty nest" syndrome is mostly non-existent and sex in midlife is better than ever. But on the whole, Rubin has the bleaker view. Traditional patterns, she says, are hard to change. Women who live vicariously for the first half of their lives tend to live vacariously in the last half. Of the 160 women she interviewed, not one defined herself in terms of her work although half had jobs and some were high-level professionals. As for the new, liberated marriages with two equal careers, Rubin finds only a smattering.

There are little glimmers of cheer in the gloom. Working-class wives shift gears more easily at mid-life because two-thirds of them are already in the labor market; "super volunteers" are the plrivileged ones -- "most likely to return to graduate or professional school . . . and forge a new career."

So who has the true story on the middle years -- Lake or Rubin? Each is telling part of it.