IF YOU'RE a member of the jet set, you probably already have it. If you're not, you probably don't need it.

And remembering J.P. Morgan's remark about his yacht -- if you have to ask how much it costs, you probably can't afford it.

We're talking about the fourth edition of "IN World Guide," the travel guide for people who have everything or those who collect status symbols (it's been called "the most snobby guide"). Published in Vaduz, capital of Liechtenstein, by IN Publishing Establishment, the book is bound in "sterling silver-trimmed black or burgundy leather" and printed on "over 1,200 pages of Bible paper" which are not fully numbered.

The price is $125.

The author is German-born Peter Finkbeiner-Zellman, described as an international traveler. His book is aimed, according to a press release, at "people who have style and taste and can appreciate and afford the utmost in luxury and service." French chef Paul Bocuse describes it as "the ultimate guide to elegant living."

It would be unfair to nit-pick and fault the IN Guide for occasional errors, omissions or certain value judgments. Few guidebooks can claim they have never listed a restaurant or club that closed its doors before the latest edition was off the press. Some readers' favorites are inevitably left out, and praise will always be given that somebody feels is undeserved.

But the book can be criticized for a tendency toward aimless name-dropping without necessarily including the "inside" information it promises. Perhaps part of the problem is the ambitious scope: the guide tries to cover history, conditions, hotels, restaurants, bars and shops in more than 50 countries, spreads itself thin, and often winds up with nothing very new or different to say. (Even Temple Fielding would never attempt such a project.)It does present a vast amount of information -- the author's far-ranging eye must be admired--but there's not a word about prices. The elite, one is left to presume, do not worry about such vulgar matters.

Finkbeiner-Zellman can guide you to drugs and sex in Manhattan, but has no revelations (spicy or otherwise) about Washington. He does assure concerned readers who plan to stay at the Watergate that "No matter what you've heard about wiretaps, it's as safe as any place for trysts." For the majority of travelers, it would be far more useful and practical to invest one-tenth of this book's cost in an authoritative guide to a specific country or area of the world.

Perhaps the most pleasurable thing about the guidebook is the way it feels and looks. Soft, dignified, classy -- rich. You don't need to read it; just stroke it occasionally and leave it lying around to impress guests. But keep an eye on it.

"IN World Guide" can be ordered from Sofaco Publishers U.S.A., Inc., 401 East 63d St., New York 10021, for $125 plus $2.50 for postage.

Eye Care, Inc., a nonprofit group that does opthalmological work for the rural poor in Haiti, says it's looking for "about 40 fearless travelers" to sign up for an after-Christmas trekking vacation across that Caribbean island, from the mountains above Port-au-Prince to Jacmel.

The one-week adventure, to begin Dec. 27 with two days of orientation and sightseeing in the capital, will include camping one night in a remote mountain village and wind up New Year's Eve on the beach. "Housing and meals will be provided by our clinics and/or affiliated organizations, except for the night in the village," says Susan W. Ruff, associate director. "The hike will be strenuous," Ruff cautions, "so participants should be in good shape."

Cost, including round-trip air fare between Washington and Port-au-Prince on Air Florida, all meals and housing, is $450 per person. "In addition," Ruff says, "we expect a contribution to be made to Eye Care . . . we would hope for $200-$300 per person." The donation is tax-deductible -- the cost of the offbeat trip is not. Haiti has "between six and 10 opthalmologists for six million people," Ruff notes.

The trekkers obviously will be in both an adventurous and charitable frame of mind, and certainly won't be primarily concerned about saving money. For comparative purposes, however, it is possible to buy a one-week Eastern Airlines tour package to Haiti this winter for a little more than $500, including accommodations in a hotel. Restaurant meals would add about $200 per person. (Of course, none of that cost would be tax-deductible.)

For more information, write Eye Care at Suite One, 523 8th St. S.E., Washington, D.C. 20003, or phone 544-4700