This is a classic set. I couldn't name a single one as they offer so many different things. This makes the list simply because Issac Asimov is a masterful writer. He puts life into his works. These books explore different faucets of branches in philosophy and math. Being that Asimov is a science fiction writer, they also explore how those branches intertwine with science. Sort of out there though, it makes for more of a philosophical series then a story set in a futuristic place.