WalMart is a great example and they are masters at this. They always, always have complimentary products within easy reach. You don't have to hunt for them. An example is premade sandwiches. Once you grab the sandwich you might want mustard or mayo. Right above the sandwiches are little bottles of each. (and they are the most expensive ones in the store)
Is it raining? WalMart has umbrella's up front. Is the local team playing a major game? WalMart has cakes decorated for the team as you walk in.
When you buy charcoal, you'll find lighter fluid, cups, paper plates, etc. really close by.
So you sell an ebook on a subject. What else do your customers need or want? Make it easy for them to pick up when they buy your ebook and I think you'll make more sales.