Including a glossary for a fantasy or science fiction epic is standard practice. Tucked away at the end of a book, they are useful references if you have set a book aside for a while and need to re-familiarize yourself with the language and culture of a story.
The first time I read the Lord of the Rings, it was a massive hard-bound copy I checked out from the Public Library. It included all three books, a biography of Tolkien and a glossary that rivaled the Silmarillion. I had a special bookmark for the glossary, and often lost myself tracing entries while assembling Middle Earth in my head.
Another epic that included a glossary at end of each book was The Wheel of Time. Here, though, the glossary tried to contain itself to the book at hand. Occasionally there was a term I was unfamiliar with, if it was not in the current book’s glossary, I would have to dig out an older book to find it, or read on, hoping the story would remind of the term's meaning.