THOUSANDS and thousands of parents and students buy them and many more read them. Guidance and admissions counselors acknowledge their value but worry about their proliferation. And colleges that have been burned by them cry foul and attack their veracity.

But the question remains: do they really have any effect on where students go to college?

"I think they have major impacts on colleges," contends Larry Mench, admissions director at Dickinson College in Pennsylvania.

"There's no question the influence of these guides is growing dramatically," said Ernest Boyer, president of the Carnegie Foundation for the Advancement of Teaching. A recent survey by the foundation found that 60 percent of high school seniors polled used the books.

The survey also found that college guides were the third most influential factor -- after college visits and parents -- in selecting a school. Guidance counselors were toward the bottom of the list.

The University of Virginia in recent years has gotten favorable write-ups in many books, including The Public Ivys. At the same time, applications from all over the country have increased substantially.

Yet Assistant Dean of Admissions Parke Muth does not draw a direct cause-and-effect relationship between the two factors. "There is no way it {the write-ups} couldn't have helped the school," he said, "but I don't think it's quantifiable in any specific way."

George Washington University also has some evidence that the guides make a difference, although in this case it was not to the school's benefit. When The Selective Guide to Colleges was first published in 1982, GWU was given two stars, on a scale of five, for its academic programs, which school officials had a "big gripe" about, recalled Admissions Director George Stoner. Later that year, the percentage of freshmen accepted who decided to enroll dropped by 25 percent from the previous year, Stoner said.

Although other factors, such as a tuition hike of 24 percent, may account for some of the drop, Stoner believes the low academic rating had an effect on admissions, "particularly in our big feeder states of New York and New Jersey."

"It's one of those things you just don't know," Stoner said. But he admits GWU is much happier now that the university has three academic stars in The Selective Guide.

In practice, the college guides can be divided into two broad categories: those that offer factual information, such as median SAT scores, majors offered, male/female ratios and the like, and those that also rate the colleges and describe such things as campus social life, the quality of the food service, and whether certain students -- such as minorities -- would feel comfortable.

"We encourage our kids to pay more attention to the less gossipy books," said Tom Scattergood, guidance counselor at Germantown Friends School, a private school in Philadelphia. Yet he acknowledges that many students do read the "gossipy" books. "It's almost entertainment," he said. "Maybe it serves to relieve the pressure."

At Bethesda-Chevy Chase High School, where about 85 percent of seniors go on to college, a survey of last year's graduates found that the guidebooks were widely used, said Cathy Gottlieb, a guidance counselor.

The survey also found that students most often used the books to find out information on such objective factors as enrollment, selectivity, programs and majors, location, admissions requirements, median SAT scores of students and cost. The students did say, however, that they would like more information on campus life.

Guidance counselors agree that guidebooks have some value in the college selection process, but that too often they are used as a quick fix instead of real -- and time-consuming -- research.

"We give students a list of 30 or 40 colleges that meet their needs, and then they have to do some research," said Robert Sortland, assistant headmaster at the John Burroughs School in St. Louis. "For the most part, the first place they turn is the guidebooks." The problem is that some students begin and end their research with such books, Sortland said. "The best guide book is the college catalogue," he said. "But it's not easy to use. It take a lot of time."

Another problem arises when students only consult one source, and that source is wrong. For instance, the 1982-82 edition of The Insider's Guide to Colleges, published by Yale students, listed median SAT scores for Dickinson College that were about 100 points too low for both math and verbal sections. Not content to wait until the mistake was corrected in the next edition, Dickinson spent about $10,000 informing prospective students about the correct scores, said Mench. He noted that surveys of Dickinson's freshmen found that two-thirds used college guides in selecting a school.

One thing no one disputes is that the college guide field is getting more crowded by the year. David Webster, an education professor at the University of Pennsylvania who has written about the guides, estimates there are now about 30 such books on the market.

They range from the general, such as Barron's Profiles of American Colleges, Lovejoy's College Guide and The College Board's College Handbook, to more specific books as The Student Guide to Catholic Colleges and Universities, The Black Student's Guide to Colleges, and The Southerner's Guide to Northeastern Colleges.

Then, there are guides to law schools, business schools and other graduate programs. And the market hardly seems to be sated. Webster recently ran into a rabbi who said he was working on a Jewish student's guide to college.

Concrete figures on sales of college guides are not available, but Webster estimates conservatively that "hundreds of thousands" of these books are sold annually.

According to the Carnegie Foundation's Boyer, guides that rate colleges and offer descriptions of campus life are filling a need that parents and students have for more information to help them through the thicket of the more than 3,000 institutions.

Another reason for the popularity of guides is that students are becoming increasingly skeptical of materials sent directly from colleges, particularly as the competition for students has heated up in recent years. Indeed, the Carnegie study found that 40 percent of the high school seniors surveyed did not fully trust the materials sent from colleges, while almost all students said the guides were accurate or very accurate.

Although many counselors grumble about the guidebooks -- particularly some subjective ones that are considered frivolous and often unfair -- some say the worst fate is not to be included at all. For instance, Dickinson College in 1982 was left out of the first edition of The Selective Guide, which by its very title confers a sort of endorsement of the colleges it includes.

"Students and parents asked us why we were not in the book," Mench recalled. Dickinson was added to the book the next year, but Mench thinks the college was hurt by its initial omission. "You can become negatively conspicuous by your absence," he added.

"There's no question the colleges have become enormously anxious about how they're described in college guides," said Boyer. He noted that many colleges use favorable reviews in their promotional materials. On the flip side, Boyer cited one college president who believes his school lost as much as 15 percent of its applications one year after being described as a "party school" in a popular guide.

Meanwhile, magazines are also getting into the college ratings act. Carleton College in Northfield, Minn., for instance, was rated the fourth-best private, liberal arts college in America in an article in U.S. News & World Report. In the past 10 years, applications to the college have more than doubled, to 2,905. "You can market all over the place . . . but you can't buy publicity like that," said John Nicholson, associate dean of admissions.

Trinity College in Texas is likewise happy with its anointment as one of "nine nifty colleges" by Time magazine. In five years, applications rose from 1,400 to 3,100. Says Admissions Director Alberta Meyer. "I can assure you when Time calls you a nifty college, it makes a difference."

Debbie Goldberg, who writes frequently on education, contributed to "The Selective Guide to Colleges" and "The Best Buys in College Education."