It might surprise you to learn that furniture is the second largest investment most of us make in our lifetimes.

A house, if we are able to buy one, is first. Furniture is second and a car is third. Even if you need furnishings for only a small apartment, the total you'll spend probably exceeds the cost of a medium-priced car.

Such information should result in most of us seeking help in purchasing this "investment," and Max and Charlotte Alth have done just that. They've writen "The Furniture Buyer's Handbook," an informative volume on home furnishings.

It's so helpful that young couples or anyone shopping for furniture for the first time would be wise to invest $9.95 in this book and absorb its useful information.

Subtitled, "How to Buy, Arrange, Maintain, and Repair Furniture," the handbook thoroughly covers its subject, beginning with some "ground rules." t

"Shoddy," which during the Civil War meant blankets and clothing that came apart when wet, is no bargain at any price. It is better to eat on fruit crates, if you have to, rather than waste your hard-earned money on a shoddy furnishing product. Some quotes:

"If you can't find what you want and need, don't compromise. Wait a while and look some more.

"One good piece of furniture is better than a number of cheap pieces.

"It isn't necessary that a room be filled with furniture, nor that it have rugs and floor lamps, etc.

"There is no such thing as 'modern' style in furniture. Buy what you like not what a salesman or your friends tell you. You are going to have to live with it.

"A good used piece of furniture is always better than a brand-new but cheap piece.

"Keep your expenditures in line with your home. A hundred thousand dollars worth of furniture in a $20,000 house looks as bad as, or worse than, a few thousand dollars worth of furniture in a $100,000 house.

"Bear in mind that furniture should be for always, so take your time, select and purchase carefully."

You get the picture that the Alths are straightforward, practical advisers.

They tell how to protect furniture from children "and other lovely beasties;" how to deal with furniture salesmen; how to avoid rip-off deals; and how to find a reputable furniture firm, besides offering oodles of information on color, fabric and basic decorating.

The handbook includes chapters on furniture for every room in the house. While so many decorating books gear their information to a particular economic group, the Alths manage to include information that will help anyone, from the person living with Early Orangecrate to Mrs. Gotrocks and her French antiques.

"The Furniture Buyer's Handbook" is published by Walker and Co., 720 Fifth Ave., New York, N.Y. 10019. The paperback edition is $9.95, the cloth edition $14.95.