The problem began around 10:30 p.m. ET and seemed to affect only ebooks published by Random House, Penguin, Simon & Schuster, Macmillan, HarperCollins and Hachette. Publishers Marketplace’s Sarah Weinman tweeted:

Reports coming in of Amazon removing buy buttons from titles by Penguin, Random House, Hachette, Macmillan, Harper, S&S. Any other pubs?

— Sarah Weinman (@sarahw) November 9, 2012

Amazon just pulled Nate Silver’s buy button. The Internet is going to explode. amazon.com/The-Signal-Noi…

— Evil Wylie (@EvilWylie) November 9, 2012

Digital Book World has a statement from Amazon: “The Kindle Store is experiencing a technical issue. We’re working to correct it.” According to some reports on the MobileRead forum, users were also unable to download Kindle titles that they’d already bought and titles that they’d added to their wishlists disappeared.

As of Friday morning, the buy buttons were restored.

It is odd that, apparently, only big-six titles were affected. Some wondered if the problem was associated with agency pricing — were only publishers who set their own ebook prices affected? But that doesn’t make sense for a couple of reasons: HarperCollins reached new agreements with Amazon (and other ebook retailers) in September, following the approval of the DOJ’s settlement with HarperCollins, Hachette and Simon & Schuster. Amazon has been discounting HarperCollins titles since then. And self-published authors set their own ebook prices through Amazon’s KDP, but those books weren’t affected, either.

It is also possible, as Nate of the Digital Reader points out in the comments, that non-big-6 titles were affected as well and nobody noticed. I have been searching Twitter and haven’t found reports of non-big-6 titles affected — and with the discussion around this topic, it seems as if those would have turned up, especially from individual authors — but it’s a possibility.

The most notorious case of Amazon purposely turning off a publisher’s ebook buy buttons happened in January 2010, when the company turned off Macmillan’s Kindle buy buttons to protest the publisher’s implementation of agency pricing. More recently, in February 2012, Amazon turned off buy buttons on nearly 5,000 Kindle titles from distributor Independent Publishers Group after IPG refused to capitulate to Amazon’s demand for better terms.

This time around, though, if the problem was indeed unintentional, we are unlikely to get more answers from Amazon about what happened.