One of the enduring wonders of these inflated, devalued times is the appearance each spring of Arthur Frommer's "Europe on $15 a Day."

You'd think the 1980-81 edition would be skinnier than the Moose Wyo., phone book, travel costs being what they are in Europe, but Frommer somehow has filled up 723 pages, advising the faithful how to do 17 major cities from London to Athens on that paltry per diem.

You also may think that Frommer, a busy executive who runs a big wholesale tour agency and manages hotels in Amsterdam and Curaco in addition to overseeing a travel-book publishing division with 42 titles this year, no longer has the time or interest to continue writing and researching the $15-a-day guidebook.

"I'm the only person who writes the book," said Frommer, bristling only slightly at a question he's obviously heare before. "I write it at home, every night and every weekend for six months."

"He writes it and edits it himself and then it goes straight to the printers without anyone else touching it," put in Paul Pasmantier, head of the Frommer book division, when the three of us met recently in Frommer's New York office.

Frommer noted that in the preface to the 1980-81 guide he has acknowledged the efforts of his associate, Nickolaus Lorey, an indefatigable Austrian.

"Nick does the initial research memorandum and comments on every For the first time this year the book carries a cover line crediting Frommer wife, Hope Arthur, an actress, who provides readers with candid tips on cultural and shopping matters (while her husband concentrates on bed and board).

What thrifty traveler since 1957, the year the first Frommer guide was published, has gone abroad without feeling the sheltering presence of Arthur and Hope, the Nick and Nora Charles of guidebook writing? I first packed along the Frommer guide, then "Europe on $3 a Day," in 1965, and within a few days of landing in Luxembourg I already was talking in the cadence nd chatty assertiveness of the Frommer style.

"Where is Hope taking us today?" my touring companion and I would ask each other, referring to the section of each city chapter where she takes over under such headings as "Hope in Stockholm" or "Sightseeing with Hope" or "Hope's Paris." Then as now, Hope also wrote the "Packing to Save Money" chapter. She knows all the tricks I've never mastered.

Frommer is an attorney who got into the Baedeker business when he served with the U.S. Army in Europe and wrote a small book, "The GI's Guide to Traveling in Europe." Army Intelligence and a yen for travel were the right ingredients for the confidential, slightly undercover approach Frommer used in the first $5-a-day guide and those that followed. Who wouldn't thrill to a book that leads you up a narrow, nondescript street to an unheardof $10 hotel room with a concierge who speaks perfect English?

In encouraging his readers to contribute their own findings, Frommer has spawned a generation of undercover travelers. Each chapter contains their verbatim discoveries, in agate type, of cheap and offbeat hotels, restaurants, shops and tours, informations the author calls "the lifeblod of the book." This may make his job a bit easier, but it hasn't stopped him from doing his field work; he has not grown too prosperous to prowl the sidestreets in search of a good, cheap cafe or, occasionally, to stay in one of his pension selections.

"I was in Madrid recently with a cuple of business executives, and I said 'Let's go to one of my places for dinner," Frommer related. "I took them to the Restaurant Valencia, at No. 44 Avenida Jose Antonio, a little upstairs place you couldn't find on your own. The check was $11 for three, and the two executives haven't stopped talking about it. Yet it was what we in the book call a 'Splurge' meal, one that costs a little more then your $15-a-day limit permits."

To stay within the Frommer formula, a couple may spend $30 in all -- $15 on the room and $1.50 each for breakfast, $2.50 for lunch, $3.50 for dinner. If the $15 room comes with breakfast, as it does in England and Holland, you get another $1.50 to spend on the dinner. This is no feat in low-cost countries like Portugal, Greece and Spain, but, said Frommer, "We had to squeeze to make the plan work in Switzerland."

Next year Arthur Frommer will title his book "Europe on $15 and $20 a day," and soon after that it will go to $20. "I had to agonize when we went from $5 to $10," he asked. "There was something special and magical about that figure $5. We're still, I think, the best-selling guidebook -- 200,000 copies a year -- and $15 sounds like a good value to most people, but to me its never seemed the same."