The Past Re-Invaded : History tweaked, and then untweaked : FROM TIME TO TIME, <i> By Jack Finney (Simon & Schuster: $23; 304 pp.)</i> : TIME AND AGAIN, <i> By Jack Finney (Scribner Paperback Fiction: $12, paperback; 400 pp.)</i>

Perhaps because it tastes of immortality, the idea of traveling through time, backward as well as forward, has always had a teasing fascination. Jack Finney’s novel, “Time and Again,” first published 25 years ago, has achieved classic status as a time-travel adventure. Now it has been reissued in an anniversary edition to coincide with its sequel, “From Time to Time,” which picks up his hero a few years later.

The sequel can be read alone, but it is more easily comprehensible and gains emotional weight if you’ve read the original first--or reread it to renew an acquaintance with Finney’s time traveler, Simon Morley.

“Time and Again” and its sequel are fantasies, wonderfully clever, original, inventive, instructive, evocative, surprising, suspenseful and, above all, charming. Albert Einstein, with his notions of rubbery time, hovers like a father figure over the stories. Why shouldn’t time be a river, with the past always there, a bend or two back, reachable if you can just figure how to cut across country, so to speak?

In “Time and Again,” Morley chances onto a secret government project, dedicated to the proposition that by steeping yourself in the minutiae of another time and place and then self-hypnotizing yourself to put out of mind all your tethering awarenesses of Now, you should be able to find yourself back there where the past is. It could be a given day in medieval Europe, in a New England village in the 1920s or in 19th-Century Manhattan. No blinking machines, just the powers of suggestion, and you are really there.

It’s the really that matters. Finney does his persuading by the accumulation of detail. Both books are successions of lovely set-pieces (illustrated with old photographs, drawings from Leslie’s Illustrated Weekly and sketches purportedly by Morley himself, a present-time advertising artist). Finney’s street-by-street evocations of 1880s New York are remarkable, the more affecting because traces of the city that was are still to be found, faint, sad and decaying.

Morley proves to have a unique gift for time-transportation, dashing to 1882 and back like a commuter. The moral dilemma, which eventually destroys the Project, is whether to tamper with history in the hope of averting a tragedy that, as the visitor from Now knows, lies ahead. The reader knows the answer, as the reader knew the assassination of Charles de Gaulle would fail in “Day of the Jackal,” but playing What If is all the fun.

In “From Time to Time,” Morley has settled comfortably in 1882, having had enough of late 1960s New York. He has tweaked history sufficiently that the crusty old physicist, E. E. Danziger, whose idea the Project was, was never born. Even those who worked in it but didn’t time travel can now remember the Project only wispily, as in a fast-fading dream. Still there are stirrings; history is untweaked and Danziger lives again, still resenting the military men who ousted him because he opposed tampering with history.

Now in the new book there seems a chance to avert World War I, a simple matter of seeing that a diplomatic courier completes a mission; there may also be a chance to divert the Titanic away from that treacherous iceberg.

“From Time to Time” is even less a mystery than “Time and Again” but it is not less suspenseful. Finney is ingenious in his manipulations of the What Ifs, building to a last ironic comment on the consequences of attempting to rewrite history. But it is the way Finney recaptures the real past that gives these novels their uncommon appeal. It is not merely nostalgia for a simpler time, although there is some of that. Finney is at pains to make clear that then, as now, it was swell to be rich and lousy to be poor, and that life was not all ragtime and parades. The appeal is in the thrill of exploring other times.

“From Time to Time,” for example, has a long and vivid set-piece on the world of vaudeville; we hear vaudevillians swapping tales of the road (bad managers, worse theaters, sudden triumphs) at their boarding houses on Manhattan’s West Side. Finney is obviously writing out of a particular private passion, and it is warmly communicated.

Finney is a very visual writer; no wonder that others of his novels have been seized by Hollywood, most notably the thrice-filmed “Invasion of the Body Snatchers.” But even in the Age of Digital Effects, reconstructing the past on a Jack Finney scale would be dauntingly expensive. These elegant fantasies may well remain literary experiences, and splendid they are, too.