SENATOR PAUL TSONGAS left public office two days ago, almost a year after the discovery of his cancer led him to decide his family is more important than his career.

Simple, right? Maybe, but in the Age of the Yuppie, Guppie, Buppie, Yumple, and other Jerry Rubin perversities, this simple, straight forward book is a radical statement. Even if Tsongas goes down in history as no more than the Alan Alda of Capitol Hill, Heading Home will have performed the invaluable service of teaching us that careerism is only skin deep--literally.

One could laud the one-term senator's pragmatic wisdom; his repeated warnings that liberalism will not in the gutters if it fails to reconcile the imperatives of business interests and social policy: his attempts to return the Democrats to the cutting edge of policy; his daring, if misguided, endorsement of Sen. John H. Glenn (D-Ohio) in late 1983.

One could bemoan the loss of a man who is smart and ambitious enough to be President, a man who could only fall prey to his slight stature, his high-pitched voice, and a deadpan wit that would make him the hit of a Harvard party but an ass in Austin.

But, as Tsongas tells us throughout Heading Home, this Mr. Smith Leaves Washington tale is not about defeat. It is in the concise prose of Tsongas' narrative, about the lessons of life, about what is important and what isn't.

IN a broader context, Heading Home is a suprisingly effective argument against many of the assumptions and priorities that shape the way we live our lives. Tsongas details the discovery of what he calls a "mild" form of cancer (Is he on the level?), his reactions; his treatment by Dr. Canellos in Boston, the reactions of his staff and friends. He wants us to feel his anguish. Not so that we will pity him, but only to jar us from our complacent attitudes towards the roles of success and career.

Once he has set us up, he proceeds to chronicle his retirement, his orchestration of public opinion (climaxing in this book) to make the world understand that it was a desire for family--not cancer--that brought him back to Lowell.

Tsongas' story is at its most feverish as it describes his reactions to a reasonable Boston Herald story revealing his cancer, but headed by the screamer; "CANCER FORCES TSONGAS OUT." He writes: "The headline was crushing. The cancer story was out, and even though the story was proper, the headline would make the lasting impression. Forced out . . . How stupid. If cancer had forced me out, I would have retired three months ago."

Tsongas' departure from the Senate should not have been a big deal ("MAN DECIDES FAMILY IS MORE IMPORTANT THAN CAREER! GIVES UP CAREER FOR WIFE, KIDS!"). Yet, the former Peace Corps volunteer in retirement at the ripe age of 43 has become a one-man spokesman against the social ravages of Yuppiedom.

Tsongas writes of how he will "partake of God's blessings" That's part true sentiment and part bathetic kitsch born of too many smiles, speeches, handshakes and babies kissed. We can, however, afford Tsongas the too noble sentiments in order to learn about sentiments that are infinitely more mundane-but more important.

"Life is a search for balance. We all have to bring the scales back to center."