Well, that depends on what exactly interests you. On the whole, for a modern English speaker, there is a huge wealth of 18th and 19th century literature available for free online, which is probably optimal because it's distant enough to offer interesting perspective, but at the same time highly readable and not so alien as to be incomprehensible without specialized study. On almost any subject that is a matter of ideological controversy today (or that was a matter of controversy in past ages), you can have a lot of fun by reading through random titles from past centuries that come out of Google Books when you search for the relevant terms.