I think this can be addressed by really thinking about your users and their profile. If these people tend to like to be informed about unique or little-known facts, they are probably fairly inquisitive. They might even like brain teasers, quiz games, etc. I think you should test this out by including one or two featured links to products that your market might appreciate.
For example, when talking about the Abe Lincoln/Secret Service example you might include further reading about Abe Lincoln with a link to the book Team of Rivals. This is a good example of an obvious subject-link related to the little-known fact. However, you could also include links to trivia games on Amazon, or general knowledge books like the World Almanac of Presidential Facts. I would include some humor in your links as well.
Take your best ideas and include a short survey in your email to get your users' opinion on the options.