Need defense? Call a magician

David Fisher has devised a trap to snare a reader which must be well-nigh escape-proof. Listen to this casual-sounding sentence, the opening to his nonfictional account of the way magic helped win World War II, The War Magician (Coward, McCann, $16.95): ''Jasper Maskelyne was drinking a glass of razor blades when the war began.''

It wasn't the broomstick, caldron, and spells kind of magic, of course, but the rabbits-out-of-hats, ladies-into-thin-air variety, and its practitioner, Jasper Maskelyne, was the son and grandson of magicians who practiced their craft in British music halls.

If you want to know exactly what Maskelyne achieved, then skip to the very end of the book for the author's summing up: ''He had caused great armies to appear on the field of battle and conjured up their weapons. He had launched fleets of ships and had vanished waterways and seemingly moved the land itself. He had done his grand illusion.'' Literally.

Most of this was done in the Middle East in the war against Rommel.

To his magic should be added a list of inventions, including fire-resistant paste, agent-007-type gadgets for spies, and an unbreakable crate for dropping supplies without a parachute.

In fact, Maskelyne was so effective he earned a great honor - a place on the Gestapo's blacklist and a bounty on his head.

The pages in between that alluring first sentence and final summing up have all the excitement of a mystery story, with Maskelyne and his group tackling their impossible assignments, producing ''tanks'' out of trucks and whole armies out of dummies, complete with fake tents, tin hats, campfires, footprints, and guns that seemed to fire.

They even ''moved'' Alexandria Harbor, blacking out the real thing and using magic tricks to ''reproduce'' it a few miles away. And the next day, giant murals, mirrors, and stage scenery convinced German reconnaissance that their bombers had been on target.

To begin with, the British Army didn't believe in magic. But then Maskelyne showed Lord Gort the German battleship Graf Spee sailing down the River Thames (all done with mirrors, of course - mirrors and a small-scale model of a battleship; just how is a trade secret). Now that was a surprise. After all, everyone knew the Graf Spee was lying at the bottom of the ocean. Besides, the Thames was miles away from the spot where Lord Gort was standing.

All the drama at Mr. Fisher's command surfaces at moments like the duel fought between magicians - Maskelyne and the personage described as ''the Imam of the Whirling Dervishes.'' The weapons are magic tricks of course, but the stakes are deadly.

In fact, there is such drama in the events themselves that Mr. Fisher didn't need to invent dialogue, as he sometimes does, or borrow a novelist's omniscience concerning a man's innermost thoughts. And while I'm talking about weak spots, I wonder why the book was not more thoroughly edited.

As for strong points, I found myself particularly fascinated by hints dropped here and there about the way magic works. I had thought it relied to a great extent on hypnotism, but in these pages, at least, that hardly seems necessary. Confusion, diversions, and false expectations worked well enough, not to mention the clever use of camouflage, disguises, fakes, mirrors, and paint that seemed to change depth and shapes.

It takes an awful lot of ammunition to dispel an illusion.

The hints are not for would-be tricksters, but for those who would just as soon not be tricked. Here, for instance, is a passage from a memo Jasper Maskelyne wrote to the British Admiralty:

''After winning the confidence of his audience, the main objective of the performing magician is to gain control of their perceptions - to make them think what he wants them to think. This may be accomplished through demonstration, subterfuge, and the necessary appreciation of human behaviour. Once this is done , the magician is free to manipulate those perceptions in any way desired. For example, if an audience is convinced a pitcher contains some milk, the entire white contents of that vessel will be perceived to be milk.''