Epistle
You'll most commonly see Hebrews called out as an "epistle, which is just a fancy word that means letters.
But at first glance, The Epistle to the Hebrews doesn't seem very epistle-like. It doesn't start off with any greetings like other letters in the Bible do, and it doesn't give many specifics about the person who's writing it or the folks it's addressed to. In fact, the whole thing reads more like a bunch of sermons strung together. The book does close like a letter in the last chapter, but those bits don't seem to fit with the rest of the book. (Source, 1149).
So while Hebrews definitely traveled far and wide, it isn't really on the same level as other New Testament letters.