Someone who's extremely proficient or knowledgeable about a particular subject can be called a cognoscente. Your deep appreciation for the books of J.R.R. Tolkein makes you a cognoscente of Lord of the Rings.
This word is often used in its plural form, cognoscenti, referring to a whole class of experts or connoisseurs: "The cognoscenti of sports disagree about who was the greatest basketball player in history." Cognoscente comes to English from Italian and goes back to a Latin root, cognoscere, "get to know." Spend a little extra time learning your word lists, and you'll soon be a cognoscente of vocabulary!