Whether someone thinks a word is "unusual" depends on that person's frame of reference, education and experience as a reader. There is not empirical list of "unusual words." When you write, use langauge that is familiar to you so that your true voice comes through.
Well some unusual words are like myriad. Myriad means you have a couple of something: for example a myriad of books or you can also use the word cornucopia, which means you have a huge quantity of something.