In the interest of full disclosure, I will admit that I am also guilty of doing this. My very first attempt at a fantasy epic included, if memory serves, about twenty pages of glossary to describe the world and its inhabitants.
That epic, in case anyone is wondering, was a terribly-written slog that never approached anything close to a resolution (or even a plot), despite clocking in at a weighty 300 pages.
But that is another story and will be told another time.
A glossary can be a useful tool for the writer. They're handy for keeping track of your world's continuity. If you, like George R.R. Martin, have a cast of many thousands, a reference sheet that lists the name of Walder Frey's third cousin can save you valuable time that could be better spent writing the scene where he is butchered and flayed alive.
That said, writers need to be careful about using glossaries as a crutch for world-building. There is a big difference between writing down a list of people affiliated with the Pajjak's court in Goragas and using your glossary to provide background information on Goragas that isn't otherwise present in the text. I've seen quite a few aspiring writers fall into this trap, where, when I say, "hey, I don't quite understand how this works or what this means," they say, "oh, well you should check out the glossary. I explained it all in there."
Don't do this. If something is important to the story, make sure that it goes in the story. You need to build your world and describe it in such a way that the important details are woven into the narrative, not stuffed into the back of the book where they are out of sight of the reader.
How do you do that? Here's some hopefully helpful tips!
Make Your Characters Memorable
I'm going to continue to flog the Martin example, because it works really well here. The Song of Ice and Fire series has a glossary listing the noble houses and the people affiliated with each house. The glossary is very sparse, listing only names, familial order (wife, sister, etc.), professions and, in very rare cases, one or two key details about the characters.
There are a lot of different characters in A Song of Ice and Fire, but Martin does an excellent job of making those characters memorable. I haven't read past book four and I haven't read any of the books in at least ten years, but I remember a majority of the principle characters, and I know enough to know the basics of the noble houses and where they stand.
Most other people who have read the series can do this as well, so much so that when I name-dropped House Fray above, anyone who knows the books (or TV show) has an idea of who and what I'm talking about.
Strive to do that with all of your characters, even the ancillary ones and the one-scene wonders. If you can describe a character well enough, through their words and actions, that they stick in the minds of your readers, that does way, way more to immortalize them than any glossary entry ever could.
Pick Good Names
There is a tendency for fantasy and science fiction writers to come up with non-standard names for their characters. This is all right, for the most part. With enough repetition, any random collection of letters and symbols can be made to stick in the reader's mind.
Try to make this easier on the reader by using either real-world names or variant spellings on real-world names. If you want to make up your own names, make sure they actually look like names and not like you threw your Scrabble tiles down the stairs and picked through the remains.
If you must make weird or alien sounding names, try to make them short and easily pronounceable.
Names that are easy to remember are easy to make memorable, especially combined with the character advice given above.
And yes, for those people who know me outside of this blog, I know this particular piece of advice is very amusing coming from someone who named a character Callenghast Mapurtaine.
Keep a Narrow Narrative Focus
Your fantasy world likely features twenty nations, dozens of cultures, and hundreds of languages and regional dialects. Including some of these in your story will make your world feel richer, larger, and more lived-in. Knowing that these diverse places will be snuffed out if the Dark Overlord wins makes the reader much more invested in the heroes' success, because they will like these things and want to see them survive.
Including all of these things in your story, on the other hand, is a bad idea. You will find that you either don't have the room to flesh everything out or, if you try to make room to flesh everything out, your story will begin to stretch to a truly staggering number of pages. Both of these approaches will only serve to mentally exhaust the reader; the first through over-stimulation and confusion, the second through boredom.
Pare everything down to its barest essentials. Is your story on the border between two of your twenty nations? Good. Does most of the story take place in one city on the border? Even better. Focus on that city. Hint that the world is larger by having one or two characters from different regions show up to offer their unique cultural perspectives.
Suggesting that there is a bigger world outside of the plot, but providing concrete detail for only the immediate setting will make your story easier for the reader to grasp, which means less time fumbling around with a glossary.
You can always use some of the other stuff in your next book!
Limit Unusual Terms
The temptation is great for fantasy authors to come up with new things. Most of the time, these new things are actually old things with different names. For example, your Goragas don't drink wine, they drink v'jaalka. They also don't fight in ritual combat. Instead, they participate in rajjakan.
Swapping terminology is fine and encouraged, as long as you don't overdo it. A little bit of this will make your world feel different and strange and wondrous, drawing the reader in. Too much of it will render sections of your book opaque to the reader. Consider this pair of hypothetical sentences:
"Velkar, the Goragas champion, took up his v'jaalka and scowled. On the morrow, he must travel to the as'bara-tlenoska, then to enter rajjakan and face Nolak at garagastarak."
Any forward momentum our hypothetical writer has tried to build up comes to a screeching halt as the reader flips ahead to the glossary to look up the necessary terms. There, the reader discovers that Velkar must travel to his ancestral homelands, where he must face his old rival Nolak in a knitting contest. Only he who crafts the finest scarf shall emerge victorious. The reader now knows what's going on, but at a cost of being pulled completely out of the story.
It may feel less evocative to replace some of the words in the sample sentences with things like "wine" "ritual combat" and "knitting," but most readers know what those things are. This makes it easier for them to focus on more important things, like reading and enjoying your story.
Define the Terms You Do Use
If you have a few terms that you have your heart set on using, make sure that you use the text to define them for the reader. You don't have to drop blobs of exposition, either--most of the time, you can infer the context of the terms in your text and get the meaning across that way. To continue with the example from above:
"I am afraid, Domasta." Velkar looked away and absently rubbed his wine cup. "I do not know that my knitting skills are such that I will defeat Nolak. I should never have answered his challenge to rajjakan."
"Do not worry," said Domasta. "There is a fire in your heart that cannot be extinguished. Surely this will give strength to your hands as they hold their needles. You will best Nolak in this duel. I have spoken!"
Our example writer has limited their terms and, from context, the sample reader now knows that Domasta, Velkar, and Nolak are people and that rajjakan is some kind of duel or challenge.
Heeding all of this advice should leave you with a tightly-focused story with well-defined terms and well-written characters with memorable traits and names. This will make a glossary no longer necessary and allow you to leave the obsessive categorization up to the people who are best suited to do it: your legions of fans who will be creating wiki entries aplenty in your honor.