It was a very clear and sustained change, on exactly that date, and the only thing I knew is that the extra sales weren't coming from people clicking on the Amazon links on the official Game Night website. (Because we get reports of those, and the rate hadn't budged an inch).
But I won't waffle further, because you've no doubt already spotted the graphic to the right. It's from the Amazon.com page for the US version of the Elvish Gene, by Mark Barrowcliffe, which came out - drum roll please - on 1st November.
So anyone who heads to Amazon.com to buy the Elvish Gene finds not only a recommendation that they might like Game Night, but a special offer too. Right at the top. Above mere recommendations for books by people like Neil Gaiman and Stephen King.
This book has been out in the UK for some time, and I noticed a while ago that Amazon.co.uk were bundling it with Game Night in a similar fashion. By then though, it wasn't selling a huge amount more than Game Night (it was a good year after it's initial launch), so I wasn't sure that it was much of a help.
But this is different. It must have been paired with Game Night right from the start, I guess because Amazon are clever enough to take recommendation information from one region to another. And as quite a big book (in the UK it was published by Macmillan), which had already achieved some success in the UK, I guess it launched with a bit of a publicity splash.
And Game Night was there to ride on its coat tales.