On Friday and Saturday, people lined up down the block to enter Semicolon Bookstore in Chicago — not for a famous speaker or a big event, but to buy books on race and antiracism.

As Americans grapple with the country’s history of racism, many of them have turned to books, propelling titles like “How to Be an Antiracist” by Ibram X. Kendi and “So You Want to Talk About Race” by Ijeoma Oluo to the best-seller lists. This surge in demand has brought a welcome lift to many independent bookstores. And after several lists circulated on social media encouraging shoppers to patronize black-owned stores, many of those booksellers in particular have been racing to keep up.

“We have had a huge financial boost,” said Danni Mullen, the owner of Semicolon Bookstore, who is black. “We went from moving 3,000 books a week to 50,000 books a week.”

All that demand, however, is becoming a challenge for some black-owned bookstores around the United States, as they attempt to manage the deluge of orders, a handful of titles that are out of stock, and occasionally, customers angered by the delays. For Ms. Mullen, she and her store manager had to quickly adapt, adding detailed instructions to the store’s voice mail and email systems, taking shifts managing their social media accounts, and at one point, unplugging the phone.

“We couldn’t answer it and answer email and fulfill orders at the same time,” she said. “We now have almost 60,000 followers on Instagram — and we’re a bookstore! We had probably 3,500 to 5,000 before.”

Ms. Mullen estimated that at least half of the money her store is making comes from the 10 or so race-related books dominating best-seller lists. They are flying off bookstore tables, retail displays and e-commerce fulfillment centers. “How to Be an Antiracist” is in its 18th reprint, according to its publisher, One World, and there are more than 1 million hardcover copies in print.

The concentration of interest among just a few titles can create long wait times, with some books on back order for weeks — and while most customers are understanding, store owners say, there are those who demand refunds or cancel their orders.

“There’s that 25 percent who will email every day asking for an order update,” Ms. Mullen said. “Well, it’s the same as it was the last four times you emailed me. It’s still not here.”

Frugal Bookstore, a black-owned store in Boston, recently put a disclaimer on its website pleading with its customers for patience.

“We understand and we apologize for the long delay that you may be experiencing with your order from our book store,” it said. “Having never operated at this scale before, we are unable to rapidly respond to your requests and we are running as fast as we can and hope you will understand the reason for our delay.”

Attempts to reach the owners were unsuccessful because, according to the woman who answered Frugal’s phone on Tuesday, they were too busy trying to get people their books.

This surge in demand could not come at a better time for bookstores, which saw business down a catastrophic 65 percent in April over the same period the year ago, according to figures from the Census Bureau. Ms. Mullen said that she was considering taking out a loan to keep her business going but now has the money to stay open for at least another year.

DeAndra Beard owns Beyond Barcodes Bookstore in Kokomo, Ind., which shares a space with a coffee shop and a learning center where students can work on English, Spanish or Haitian Creole, among other languages. Those other businesses have always been her main sources of revenue, Ms. Beard said, and they have all struggled since the spring. But books aren’t secondary anymore.

“I have a 13-year-old son who is tracking all the states we’ve had orders from, and I think he said to me yesterday that all we have left is West Virginia,” she said. “The first order I got from Hawaii, I wanted to cry.”

While the business is welcome, it can be frustrating for booksellers that the interest is so narrowly focused. Ms. Mullen has been trying to steer customers toward African-American fiction like “Parable of the Sower” by Octavia Butler, “The Vanishing Half” by Brit Bennett and “Kingdom of Souls” by Rena Barron.

“I think the way to become more antiracist is first to be empathetic,” Ms. Mullen said. “And to first be empathetic, read our stories.”