Biographies are big. Book readers watch a lot of television, too. Older people don't do much reading. But 10 percent of Americans are bookworms, claiming to read at the rate of more than 50 books per year.

Those are among the revelations of a national survey of book reading habits conducted last May and June through hour-long interviews with 1,450 readers and nonreaders. All of the respondents were at least 16 years old.

The survey was conducted by Yankelovich, Skelly and White for the Book Industry Study Group, Inc., a publishing industry organization, and some of the results were announced yesterday at the Library of Congress.

The respondents were divided into "book readers" (the 55 percent who said they had read at least one book during the six months prior to the interviews), "non-book readers" (the 39 percent who said they had read magazines or newspapers but no books during that period) and "non-readers" (6 percent).

Among the findings:

Biographies and autobiographies were the most popular books. Among book readers, 44 percent said they liked to read biographies/autobiographies, and 29 percent claimed to have read one in the last six months. "Action/adventure fiction" was the next most popular category, closely followed by "historical novels." "Mysteries/detective stories" were in fourth place. Fifth place was a toss-up between "short stores" (34 percent liked to read them but only 21 percent had done so) and "cookbooks/home economics" (29 percent liked them, 22 percent had read them recently).

"Languages" books were the least popular catagory overall, and "Westerns" the least popular fiction genre.

Watching television was a major leisure activity of 79 percent of both "non-readers" and "non-book readers," but 76 percent of the book readers watched the tube, too. In fact, book readers said they listened to the radio an average of 16 hours a week, watched TV an average of 15 hours a week, and read an average of 14 hours a week. Non-readers said they watched TV an average of 24 hours a week and listened to the radio an average of 17 hours a week.

Three-fourths of the book readers were under the age of 50. At a press conference yesterday, a representative of the polling firm cautioned people not to postpone reading all those great books they've been setting aside until their retirement. "One's eyesight is not kept indefinitely," he said.

A majority of respondents characterize their reading habits as consistent or "on and off" over the years. But 18 percent said their heavy reading was confined to school years, and the number of those who said their reading was decreasing was larger than those who said their reading was increasing.

More women than men read books - 58 percent of the survey's "book readers" were women. (52 percent of all respondents were women.)

Slightly more than half the respondents said they had bought a book within the last six months. Slightly more than a quarter had used the library (the library-using book readers had checked out an average of two books per trip, three trips per month). The number of book readers who had read books given or loaned to them by others almost equaled those who had used the library.

When book readers read, they were likely to do it for "pleasure." Non-book readers were relatively likely to do their reading for "general knowledge." Women were more likely than men to read for "pleasure."

The likeliest place to find a book reader was in the living room, den or bedroom in the evening hours.

Book readers who didn't have a great deal of time said it was as easy to read a few pages of a book as to read a magazine or newspaper. Non-book readers disagreed, saying it's difficult to read a book without large chunks of time.

Book readers were more likely to be parents than non-book readers or non-readers were.

Asked to name their "last book read," those polled named 750 titles, and only about a quarter of those were best-sellers. Forty-five percent of the "last books read" were purchased, for an average price of $4.70.

Officials of the polling firm were asked how they could be sure that people weren't fibbing about their reading habits. "We are hard-bitten skeptics," replied executive vice president Arthur H. White. Interviewers asked for specifics on titles, prices and places of purchase or loan, he added. They results may still be "overstated," but at least they reveal "a desire to read."

Those interviewed could define for themselves what "reading a book" meant. No pop quizzes were given on the material to make sure every page had been read.