Twenty years... That's how long Amazon has been selling books now. It's also how long we've been buying books from them, as we were amongst their earliest customers – I placed an order just a few days after their online store first opened.
Here's one interesting little chart I stumbled across: the top 20 all-time best sellers on Amazon. I'm surprised that the earliest publication date for any of these is in 2006. There are only a few of these that I've read: Harry Potter, and the Larsson books. I bought "Gone Girl", but haven't read it yet.