Three people were already waiting outside the old Pension Building at 5th and F streets Northwest when the security guards, arrived at 6:15 yesterday morning. Before 10, when the day's business formally opened, the number had grown to more than 500, and the first 100 held numbered slips of paper that certified their places in line.

There was an eerie silence when all those people began pouring in, running to their favorite spots among the tables loaded with 100,000 books arranged in 40 categories. "Humor' shared a table with 'U.S. Government," "History" with "Mystery," and a dozen foreign languages were heaped on four tables in the middle, including a two-volume Italian translation of Dostoevski's "I Fratelli Karamazov ."

The 12th annual Goodwill Industries Book Sale was under way, and the Pension Building had become an enormous, temporary second-hand bookstore.

Those at the front of the long, orderly line had included a high proportion of dealers -- more than 30, from as far away as New England, who would stay at the book sale for two or three days. Others were collectors who would zero in their specialties, sweep through the tables with an expert eye and fill several cardboard boxes in a few minutes.

Cookbooks, in the far right corner, were deliberately mixed with gardening, one attendant explained, because "otherwise someone would just come in and sweep them all into boxes -- whoosh! -- without even checking the titles."

"I hope the chess books aren't all gone," said late-comer James Cope, president of the Capablanca Chess Club. He hurried toward the hobbies section and 10 minutes later emerged with a dozen chess books and one called "The Art of Sensual Massage." "I'll give some of the chess books to young players I know who are hoping to become experts," said Cope. "Personally, I'm planning to become an expert on massage."

By noon, customers had filled knapsacks, brief cases, boxes, shopping bags and duffel bags and began forming a dozen lines where they would spend half an hour before reaching the cashier. Many read; others chatted about books. "The prices are good," one woman said, "but not as low as they used to be."

Collector Oscar DeWitt reached into his box and came out with "Trout Madness," which had $4.95 on its dust jacket but was marked up to $6 for the sale. "I guess that's okay. Maybe it's out of print and hard to get now. I don't know -- I just decided I was willing to pay $6 for it."

DeWitt is one of the customers who has been coming since the first Goodwill Industries Book Sale in 1968, which had only 3,000 volumes. Last year's sale netted $70,000, and this year's -- which will last for six days, with offerings culled from about 750,000 books given to Goodwill during the past year -- is expected to do better. Old customers have noticed some other changes, too.

"It's a lot more competitive in the last few years than it used to be," DeWitt said. "There are a lot more collectors coming, and the prices are going up. It's still a lot lower than most bookstores, but higher than it used to be. There used to be a lot more books for under $1."

Before the sale opened, dealers could be seen peering in, trying to read the titles and spot a rarity that they could scoop up in the crucial opening minutes.

"A lot of people are here because they hope they can outguess us or we will make a mistake," said Lois England who is in charge of the enormous, year-round work of sorting and pricing the books -- which takes some 16 volunteers two or three days a week. "We do make mistakes -- that's what brings in the dealers. But we've developed some experts through experience. We have a lot of reference works that tell us how to price a book, and when something looks like it may be valuable we send it to a professional appraiser."

In a storage area, hundreds of boxes of books are kept in reserve, to be brought out and upgrade the stock again after it has been picked over. Maggie Booda, chairman of this year's book sale, came in wearing a blue apron with "The Boss" embroidered on it, rested for a moment against a stack of boxes and sighed: "Everybody in the place has a bad back now but me. I got a bad back last year and don't plan to get another."