Keep up your car yourself; Reader's Digest Complete Car Care Manual. Pleasantville, N.Y.: The Reader's Digest Association Inc. $22.95.

As the cost of buying, running, and repairing an automobile soars with the birds, more and more motorists are opting to "fix it themselves." It will cost less money and the job may be done right the first time.

Too, there's the matter of satisfaction that accrues to the do-it-yourself, despite the dirty hands.

What the novice needs is a good car-repair book that will take him through each job a step at a time, one by one by one. While there are a number of well-accepted manuals on the market shelf, one of the best is this Reader's Digest car care manual, a compendium that runs nearly 500 pages, including an index and glossary of basic terms.

Reader's Digest says that if you spen d $8,000 for an average 1982-model midsize auto and you drive it for 100,000 miles, you'll spend another $10,000 for maintenance and repair.

No other household purchase except a house costs more.

Compare the costs:

* Replacing the oil and filter on an average car costs around $22 if you let a mechanic do it. Do the job yourself and the cost is far less than half of that.

* New shocks, if replaced in a service shop, may run to $50 or more. But do the switch yourself and the cost drop below $20.

* A new muffler runs to $60 or $80 in a shop. AGain, you can do the job for under $20.

* A minor tuneup runs to $50 and up; do it yourself for under $20.

What the Reader's Digest manual does is take the motorist -- novice as well as the experienced home mechanic -- through each job step by step. By following directions, you shouldn't have too much trouble, because each job is rated by degree of difficulty right at the start.

A one-wrench job, for example, is designed for a novice with no previous car experience at all. A five-wrench job is at the high end of the scale and should only be done by an advanced amateur mechanic with extensive car-repair experience under his belt.

Testing and replacing an electric fuel pump on a car, for example, is rated as a three-wrench job and should take from one to three hours to do. Checking the automatic-transmission fluid is a one-wrench job, while changing the automatic transmission fluid and replacing the filter is a four-wrench job. Get the idea?

Installing new spark-plug calbes is a two-wrench job; but inspecting the plugs is a one-wrench job that takes only a few minutes of your time.

The Digest's manual is a course in basic automotive engineering that can be understood without an engineering degree.

Two years in the making, the project was under the direction of Wade A. Hoyt, a member of the Society of Automotive Engineers and past president of the International Motor Press Association.

Every car-maintenance job in the book was actually performed by the Digest crew and not just written about at arm's length. A rented garage in Tarrytown, N.Y., was turned into a photographic studio. Then engines, transmissons, stripped chassis, and whole cars were brought into the garage and the repair jobs photographed from start to finish.

Major sections include: (1) the cost of driving a car, including buying, selling, car loans, and economy; (2) how a car works; (3) major components of popular cars; (4) tools and their uses; (5) maintaining a car; and (6) how to repair a car.

The 480-page manual includes a separate 72-page book on tuning up 1972-81 cars by General Motors, Ford, Chrysler, and American Motors and 1972-80 data for 11 imported cars.

Even a couple of oil changes will more than pay for the cost of the book.