If you're one of those late-night Agatha Christie readers who appreciates an occasional twisted ending, then hold your glass especially high tonight: You've just gone through a vintage year.

At least so far as the U.S. economy is concerned, 1978 has been the year of surprise and perverse outcomes. If the New Year brings a little predictability, surely even the most restless among us won't complain.

This was the year for example, that President Carter, recalling his 1976 campaign, proposed a major "tax reform" program-only to see Congress not merely reject it, but also undo previous years' "reform" legislation as well.

It was the year that inflation-which was running at about 6 percent at this times last year-shot through the roof, instead of continuing steady as expected. Prices recently have been surging at an 8 to 9 percent pace.

And it was the year that the once-mighty dollar plummeted through the bottom of the major foreign exchange markets, confounding a good many domestic analysts and forcing the administration into a major rescue operation.

It was the year that:

President Carter proposed what he called a "tight and lean" fiscal 1979 budget-with a $60.6 billion deficit-only to be upstaged later by congressional tinkering that brought it to $38.9 billion by luck and changed conditions.

The administration, worried about continued high unemployment, began the year by proposing a sizable $24.5 billion tax reduction, insisting it was needed to stimulate the economy. But within five months, the White House had to switch its signals. To everyone's surprise, unemployment was coming down more rapidly than expected. Instead, Carter trimmed his tax-cut package to combat inflation.

The White House proposed a new comprehensive energy program that Carter described as "the moral equivalent of war," but found itself having to accept a token victory to escape full surrender at the hands of Congress.

The oil-exporting countries socked the world with a new 14.5 percent price hike for 1979-despite careful courting by the U.S. and widespread official predicitons here that the rise wouldn't exceed a modest 5 percent.

Congress began the year rushing pell-mell to roll backthe big Social Security payroll tax increases it had voted the month before. But the effort lost ground after constituents showed they were more worried about the budget.

Carter replaced Arthur F. Burns as chairman of the Federal Reserve Board, only to see his successor-moderate Democrat G. William Miller-preside over the sharpest rise in the interest rates in years.

The White House began 1978 eschewing any form of direct jawboning, only to end the year with the most elaborate set of "voluntary" wage-price restrains since the Nixon era controls.

The administration issued dire predictions about the impact of the first quarter's coal strike on the economy. But they never came true, even though the strike lasted through March. Later, officials hid their paperwork.

Asserting it was worried about the plight of the middle-income taxpayer, Congress took $2.6 billion from the kitty it had planned for general tax-cut measures and diverted it to capital gains cuts, which mainly benefit the rich.

The nation continued to make significant strides in saving energy, even though Congress steadfastly refused to enact the president's energy proposals. Auto fuel and industrial energy conservation improved.

Events ultimately made hamburger out of analysts' predictions that beef prices would moderate in late 1978, Earlier signs that cattlemen might be rebuilding their herds proved false. Instead, prices climbed sharply.

Proposition 13 fever gripped the country after a tax-rollback victory in California but eventually fizzled as local communities realized there was more at stake than simply staging a protest. Referenda showed mixed results.

Supporters of the Humphrey-hawkins "full employment" bill, frustrated because President Carter wouldn't support the measure, pressured him into endorsing it-only to find the measure was meaningless as passed.

Aided by the Catholic school lobby, private-school forces almost succeded in pushing an expensive tuition tax credit bill-losing the battle after parochial school officials declined to compromise at the end.

The stock market once again rose when it was supposed to decline and declined when it was supposed to rise-proving that it is consistently inconsistent in the face of changing economic developments.

The administration started the year with a bold new thrust-with a claim that, after spending its first 12 months in disarray, the White House finally was getting its economic act together.

Carter proposed a relatively modest budget, with continued big job-creation programs to reduce unemployment. He also unveiled a major "tax reform" package, designed to cut taxes and close "loopholes" that benefit the rich.

But the president apparently misjudged the voters' mood. Congress not only was cool to the Carter package, but beganseeking alternatives it thought would please upper-middle-income taxpayers.

The result: The president's program essentially was scrapped, and the lawnmakers voted instead to slash capital gains taxes. They also undid several key provisions of the 1976 Tax Reform Act.

Meanwhile, inflation accelerated more rapidly than anyone had predicted, with prices finally returning to double-digit pace. First, food prices proved more troublesome than expected. Then other prices went through the roof.

Although Carter did trim the size of his tax-cut package some, White House political advisers still seemed unconvinced that the anti-inflation effort needed strengthening. The administration went through several false starts.

Finally, the Federal Reserve Board took over, raising interest rates to dampen loan demand, despite White House protests. The irony was that it didn't quite work. The Fed, too, seemed unable to get a handle on inflation.

Eventually, in early autumn, the administration brought out a new "voluntary" wage-price guidelines program-an elaborate, but well-crafted, plan that still must be tested in the next several months.

What brought everything to a head, however, was the sudden decline of the dollar. The U.S. currency had been sliding for several months. But fears of new U.S. inflation sent it plunging sharp. The White House had to act.

The result was a November dollar-rescue plan that sent a jolt through the markets and finally convinced at least a substantial number of analysts that the White House was getting serious about the anti-inflation fight.

For 1979, Carter is preparing to propose a significantly tighter budget, but those plans could be sidetracked if the nation heads into a recession, as predicted.It's too early to tell how the anti-inflation effort will work.

All things considered, the most certain thing that can be said about 1978 probably willtake place tonight: After 12 months of constant surprises, the year is coming to an end. CAPTION: Picture 1, Rep. Al Ullman (left) and Sen. Russell Long confer before a hearing in July on energy tax bill. By James K. W. Atherton-The Washington Post; Picture 2, A West German Bundesbank foreign exchange dealer keeps track of the U.S. dollar in February. UPI*