There are many themes when one considers Tolkein's book, The Hobbit. The theme of friendship, of duty, honor, bravery, and of people underestimating someone based on their size, is always good fodder for an essay. There's also the fact that Gandalf knew the danger that he was offering and instead, withheld that information. Then there's the entire theme about the corruption of wealth. The dwarves were horribly rich under the mountains, so much so that they cut themselves off from the rest of the world. It didn't end so well, did it. Then you could also write on the symbolism of Smaug. LOADS of things to use for an essay.