The victory of Shirley Williams in the Crosby by-election outside Liverpool last week made her the first person claiming membership in Britain's new Social Democratic Party to win a seat in Parliament. In trouncing the Conservative and Labor candidates in what had been historically a Tory stronghold, she became the 24th SDP member of Parliament, the others all having switched from the Labor or Conservative benches.

But the election meant much more than that. It confirmed the public opinion polls showing that the SDP-Liberal alliance that supported Williams now commands such a strong plurality in Britain as to gain a majority in Parliament if a new election were being held today.

When Prime Minister Margaret Thatcher actually calls that election, in the autumn of 1983 or the spring of 1984, it is very likely that Williams' victory will be seen as a landmark in the political realignment of that nation. And it is far from impossible that the former Labor minister of education could become Thatcher's successor.

So it was not just academic curiosity that set me last weekend to reading "Politics Is for People," the book Harvard University Press published last August as a much-expanded version of the Godkin lectures Williams delivered in Cambridge, Mass., in 1980. Nor are her thoughts pertinent only to an understanding of what is happening today in British politics. Much of what she has to say applies to our own situation.

The monetarist elements of President Reagan's policies have produced an unexpected recession in this country just about as quickly as they led to a planned recession in the first year of Thatcher's government--and for the same reasons.

"Tight control of the money supply in an imperfect economy," Williams writes, "hits investment and employment harder and sooner than it hits inflation. When interest rates increase, firms put off new investment; firms unable to finance working capital go to the wall; moreover, higher costs are often passed on in the form of higher prices. . . ."

"Tight money," she says, "similarly hits jobs long before it hits wages. Organized labor is often strong enough to resist and delay any attempt to drive real wages down."

This may be less true in the United States than in Britain, she concedes, because of the relative weakness of American unions. But in either country, "real wages will only fall when unemployment has gone so high that it has seriously undermined the unions' bargaining power."

That analysis has been borne out by events in Britain since the Thatcher government came to power. Last week The Economist proclaimed that the Thatcher recession had ended, because the gross natioal product rose by one-quarter of 1 percent in the July-September quarter.

But that was no help in the Crosby election, because unemployment was at a record postwar high and the economy was still more than 7 percent smaller--in real terms--than it had been when the Tories came to power in the spring of 1979.

There are enough differences between the Reagan and Thatcher programs and between the American and British economies to make anyone cautious about predicting a similarly bleak picture here when the Reagan economy arrives at its third birthday.

But there are also enough similarities to make Williams' book a cautionary tale for our own conservatives and a stimulating treatise for those Democrats struggling to find sensible alternatives to the Reagan policies, if they prove to be a failure.

Williams has some plausible notions about industrial and technological innovation, about job training and the alternation of education and employment throughout a working life, about the encouragement of small business, the democratization of unions, the application of incomes policy, the improvement of schools and about the return of social services to the community level. All of them are challenges for us no less than for Britain.

Most refreshing of all, she brings a humane intelligence to these topics that makes it easy to see why she is so popular on the hustings.

The same combination of qualities that is bringing a new party to birth in Britain could contribute to the rebirth of the oldest party here