Some things to know before buying a new car; How to Buy a Car, by James R. Ross. New York: St. Martin's Press. $3.95.

It's one of the most costly purchases the average person ever makes. After his home, buying a new car is a reallym big-ticket item these days. In fact, he may spend far more money on buying a new car these days than he paid for his house 12 or 15 years ago.

Thus, buying a new car is a matter of great importance to Mr. and Mrs. America.

Despite its importance, however, many a motorist is highly inept when it comes to negotiating a new-car purchase to his own advantage and not that of the car dealer's. Simply, the car buyer just doesn't know enough about the auto business to know when he's being taken for a ride and when he's getting a fair deal.

Novice car buyers lose up to $4 billion a year, according to James R. Ross, a former car salesman with more than 5,000 hours of sales experience invlving more than 2,000 personal contacts with potential car buyers.

By knowing some of the tricks of the trade, he'll be in a better position to talk to the car salesman or dealer.

That's the thrust of this 146-page paperback, low on size but high in know-how.

Indicating the risk to the car buyer are the tens of millions of car, both domestic and import, which have been recalled over the last 10 years. In short, laying out $8,000 to $10,000 for a new car is, in some instances, a gamble.

The book is filled with dozens of hints on how to increase your negotiating edge and bargaining power, according to the cover of the paperback book. Chapter 8, for example, has the provocative title of "Handling the Salesman."

"Salesman are trained to use a system to sell a car: either a canned presentation or a selling cycle," asserts the author.

"A salesman is trained to control his customer by asking questions. He is trained to answer question by asking questions, and to seek a commitment to purchase by answering a question with a question that asks for the sale.

"If you are to control the salesman, you must learn to use the question principle."

Simply, buying a new car is like playing a game. The only problem is, the two opponents are not on an equal footing, by any means. The salesman is trained to stay well ahead of the potential buyer. He is trained to make the correct moves. Few car seekers are so well prepared.

This is not to say that buying a new car need be a bad experience at all.But it is to the advantage of the motorist to know what the car salesman is up to -- and to understand in some way just how he thinks.

This is what author Ross has tried to convey in the book.

Whether or not he has succeeded will depend on the reader's experience when he next drops into a showroom with the idea of buying a new or used car.

As part of his crash course, Mr. Ross gets into the financing of a vehicle, how to approach the service department after you've bought a car, and what to do if you're dissatisfied with the car afterm you've bought is and you can't get any satisfaction from the dealer.

At $3.95, it's had to fault the price considering the cost of the end product -- if you buy.