Up to 50 million American taxpayers may be eligible to open Individual Retirement Accounts IRAs), personal retirement plans made possible by changes in the tax laws in the 1970s.

But as of the end of 1978, according to Treasury Department figures, only 2.7 million IRA accounts were opened.

No one knows for sure why only 5 percent of persons eligible have started an IRA. But a Northern Virginia resident who started to look at this situation stumbled over a number of possible reasons and decided to share his experience with others in the form of a new handbook.

Don Carmelo's The ABCs of IRAs (Cisco Inc., Box 2400, Arlington, Va. 22202, $6) is his first publishing venture. It is a valuable guide that answers most questions and provides a list of sources for further details.

When Carmelo began his search for IRA information, he discovered Internal Revenue Service Publication 590 and several books, all "written in legalese."

After another week in a law library, "I began to suspect that I had the answer to the question of why more people who are eligible for IRAs don't open them," he recalled. His answer: IRAs are both misunderstood and not promoted,

Some experts maintain that IRAs don't have a popular following because of contribution maximums of $1,500 or $1,750. But tax return data show that higher-income taxpayers are the only ones who open IRAs in significant numbers. What do they know that the rest of us don't know?

Carmelo concludes that two factors are at work:

IRAs are complicated and most people don't understand them, while those persons in higher income brackets have an incentive to start an IRA. These individuals normally are more knowledgeable about financial and tax matters.

No one is actively selling IRAs. Insurance sales people backed off because of low sales commissions and the prohibition of fixed premium annuities for IRAs. Only the savings and loans and no-load mutual funds actively promote IRAs, but the mutual funds have the obstacle of trying to sell something complicated by mail and S&Ls "aren't sure how they feel about IRAs," Carmelo contends. Some S&Ls want them and some don't and individuals who go to S&Ls for information often are disappointed.

While the intent of Congress was to provide a simple tax-deferral that could be used to build retirement savings, the rules, requlations, changes and interpretations just grow and grow. Carmelo's book helps fill the information gap with chapters on different kinds of IRAs, tables on how savings grow, varieties of IRA investment vehicles, nuts and bolts on how to start an IRA, details on how withdraw funds, how IRAs are taxed, employer-sponsored plans and IRA offices to call toll-free for more information.

More recently, Congress enacted sweeping changes in federal student aid programs for college-level education.

Although the Higher Education Amendments of 1980 were passed in September and signed into law by President Carter on Oct. 3, telephone callers to the Department of Education and to financial institutions in recent weeks have generally received uninformed responses when questions are asked about the new program. Bankers and bureaucrats alike have asked the callers, "What law? What changes?"

Briefly, the measure will authorize $50 billion of tuition assistance over the next five years. Washington Post columnist Jane Bryant Quinn, in two articles about the legislation published last month in the Washington Business section (Mondays Oct. 6, 13) concluded that the new grant rules will penalize the prudent and reward spendthrifts.

Nevertheless, Quinn advised, parents should use the new rules to their best advantage. Money saved or expenses deferred for college educations can play an important part in long-term financial planning.

For one thing, home equity no longer will be one of the factors in assessing a family's contributions to college costs.

But where do you go for information? If you can wait, school guidance offices should have more complete infromation on basic grants available after the first of the year. However, many counselors cannot keep up to date with the complexity of current developments and they often steer students and parents to published documents from the government or to the College Board. Eventually, bank officials at institutions engaged in student loans will have details.

A more immedidiate source is a series of booklets published last month by Octameron Press, in Alexandria. Robert Leider, a financial aid specialist and author of one booklet, said the new law "contains something for everybody." aFor example, there is a new parent loan plan under which parents can borrow $3,000 a year at 9 percent interest (up from 7 percent) for each student in college. Loan limits were increased to$25,000 from $15,000 for graduate work and to $12,500 from $7,500 for undergraduate work. And part-time students will be eligible for assistance, for the first time.

Leider says that students and parents could overlook as much as $15,000 in federal aid from their planning, if they don't know details of the new law.

To assit in planning, Leider totally revised his guide, Don't Miss Out (Octameron Press, Box 3437, Alexandria, Va. 22302, $2.25), within 24 hours of passage by Congress. The 56-page publication outlines various strategies for reducing the cost of college, including federal aid.

There also are two smaller, companion booklets: Help From the Governor & Locating Lenders (1.25), outlining contacts for scholarships and student loans, and Am I Eligible ($1.25), which is a guide for families to determine the eligibiltiy and size of student aid likely, based on their financial circumstances.

Leider cautions parents and students not to limit their search for financial aid to federal loans and grants. He notes, for example, that with the end of the baby boom, many colleges are competing fiercely for academically talented students. About 50,000 academic scholarships are offered by 700 colleges and universities and "most of these awards are not based on financial need," he says.