Do you know anything else about your audience other than that they enjoy "daily facts"?
The more relevant your affiliate ads, the higher conversion rate you should have, so target with that in mind. Obviously, the more you know about your audience, the closer you can target.
It seems like something like this could initially go after books with trivia info, mental floss, etc. But I wouldn't get too caught up with this step. It's a daily email, so you could try out 365 different things each year and see what works.
Your best bet is to just play around with products, measure everything you can (obviously MailChimp clicks, but you must also measure conversions after they click, revenue, etc).