Linguistics is the scientific study of languages involving the analysis of a language's form, meaning, and context. This is traditionally done by observing the interplay between sound and meaning, but with time we have developed or discovered various new methods to make it easier, such as phonetics.
Although not many might be able to differentiate between a linguist and a polyglot, the former is gaining popularity amongst youngsters. Be it developing a new language for popular Movies or TV show, such as Dothraki for the show Game of Thrones, or studying the structure of a highly popular constructed language such as Esperanto, linguistics find its use everywhere.
Due to its increasing popularity, which is caused mostly due to increased use of constructed languages in popular culture, the demand for books related to linguistics or the study of languages is increasing. This presents a great opportunity, as you can now sell or rent your old books and earn some quick hard cash.
So let’s say if you have already learned all of Tolkien’s Elvish languages or are now fluent is Esperanto and no longer need the books you had bought for it, you can put it up for sale and who knows, in a few months you’ll have one more person to discuss LOTR in Elvish with.