Discover and explore 21 European cities on foot; Turn Right at the Fountain, by George W. Oakes, with Alexandra Chapman. New York: Holt, Rinehart & Winston. $8.95

George Oakes's updated version of "Turn Right at the Fountain" reads something like a treasure map, inasmuch as one needs the map not for its own sake but because it's the key to the "X" that marks the spot. And discovering "X's" is what this book is all about.

Mr. Oakes's walking tours are organized collections of "X's" in 21 major European cities, from London to Edinburgh to Paris, across the Continent all the way to Athens. Though he opens with a plea for the "walking" cause, his automatic assumption is that his reader wants to see a European city on foot, for either economic or aesthetic reasons.

It was for both reasons that I walked incessantly during a time in Paris last year. No, I didn't have "Turn Right at the Fountain," and, in retrospect, I wish that I had. Wandering the streets is fun, but it takes time, which can be a limited commodity. Launching out for the Luxembourg Gardens one morning was a bit like leaving port without knowledge of currents, and my flapping street map made it definitely difficult to steer. This collection of diagrammed guides and accompanying texts fits into one paperback book and goes into one good-size pocket when not needed. Pull it out, and you have a current composite of travel brochures for the 21 cities, telling names of museums, exact positions of palaces, the times they're open and not crowded, what bridges you cross to get there, and gardens to peek into along the way. Finally, each tour includes an often overlooked detail -- how to get back to where you started once it's over.

This collection of tours was orginally written up, city by city, by the same author for the New York Times. The series was so successful it was put into this book, which has gone through four editions. Here are the latest facts on what there is to see in these chosen cities.

Being brochurelike, the commentary will freely tell you what's "important," "fascinating," or "charming" along a particular walking route. The morning I was meandering through Paris, after having finally found the Luxembourg Gardens and in the process having run across a Picasso exhibition in a "mystery" museum, I wandered down a street into a church labeled St. Sulpice. Inside an unseen organist practiced one measure of Bach over and over while rays of sun shot through stained glass to strike a polished wooden crucifix. I admit to a small voice in the back of my mind that said, "This is magnificent! Uh, I wonder how it rates?" Pure appreciation hastily stepped in and stomped out the question, but when I found a copy of "Turn Right at the Fountain" I turned to the page covering the Eglise de St. Sulpice.m There were the words in answer to my wondering: ". . . second largest church in Paris. It's organ is world-famous."

"Turn Right at the Fountain" may not offer you a treasure of sparkling prose, but that may be hardly what you need or want as you follow Mr. Oakes's arrows, taking care not to stumble on any time-worn precious stones while crossing the Acropolis.