I'm busy reading A Connecticut Yankee in King Arthur's Court by Mark Twain. I've never read it before, I must have missed in my reading travels. Anyway, as I'm reading it, I began to think of all the things a person show know, in order to get full enjoyment from the book. So here goes. Please understand, I'm in the middle of it, so the list may not be complete.
The King Arthur Legends
It would be good to know at least a little something about the legends of King Arthur, Queen Guinevere, Merlin & the Knights. It is surprisingly adult in character - lots of character flaws & tragedy. Also lots of magic and mystery. These legends have been around for centuries upon centuries, but a good and easy start for a beginner would be T.H. White's Once and Future King. It's a pretty thick book, lots of pages with lots of words, but you get a good overview of things. One of the more famous series of books is Sir Thomas Malory's, Le Morte d'Arthur but be aware, this one is not for the faint hearted. It was written back in the 15th century or there abouts, and it is not an easy read. At least not for me, back when I tried to read it once.
You should also know some British history. Broad surveys are better than nothing, but the more detail the better. At the very least you should read something that covers the period from the withdrawal of Rome from Britain to Edward the Confessor - that's a period of 600 years or so. And Twain also mixes is epochs a bit, so you might as well continue on until at least the early part of Henry VIII's reign. Pay special attention to Henry II. That's another 500 years, give or take. It helps to know about civil and religious institutions in place during these times. The most powerful institution in Europe during most of this period was the church, and though much of what I read so far, the protagonist expresses fear of the church. Altho I haven't seen Feudalism mentioned by name in the book, it sure is described in actions, and that was an institution that was around in one form or another for quite awhile. It would help to be aware of the different strata of medieval British society, from the Monarch on down to the Serfs.
19th Century World History
The thing is, Mark Twain was not re-writing the Arthurian legends. He satirizes them a bit, but he's also taking his shots at the world he knew, both in North America and Europe. You should know something about the Industrial revolution, especially in the USA, but also in the world at large. You should know a bit about the politics, institutions & attitudes of the late 19th century, both in the USA & Europe. And also the technology. It was a very innovative time.
Have a sense of humor
Mark Twain writes some of the funniest things I've ever read, and they're just thrown in here or there. This is where the 'bejabbers' in the title comes in, it was used in a couple of paragraphs in which Twain pokes a little fun at the literary world. I'm still chuckling about it, but then my world may be a little insular than most (sigh). Sometimes his humor is very obvious. Other times his humor is very subtle, and you have to be on your toes to catch it, which is why the more awareness you have of his culture or what he's writing about, the more likely you are to really understand. This is turning out (I believe) to be kind of a 'dark' book, not a happy story at all, but the biting humor is still there.