Apart from the level of vocabulary - whether a set of words is too simple, average or too advanced for a corresponding set of readers - there is another approach to vocabulary which is important for writers.
This is the range of the words used, by which I mean the lexicon of a particular genre or type of book.
Tolkien was particularly careful in his choice of words, being a philologist by profession. His most famous work, The Lord of the Rings, contains several sub-lexicons in the sense that there are a number of different ranges of words used depending upon the people speaking them and the circumstances in which they are used. We have the colloquial speech of the hobbits, the book’s heroes, but counterpoised with that is the more formal lexicon of the ‘nobler races’, as in this example from the third volume of The Lord of the Rings, The Return of the King, in which Gandalf, a wizard from the Blessed Lands of the West addresses Denethor, a Steward of an ancient but crumbling realm:
‘Unless the king should come again?’ said Gandalf. 'Well, my lord Steward, it is your task to keep some kingdom still against that event, which few now look to see. In that task you shall have all the aid that you are pleased to ask for. But I will say this: the rule of no realm is mine, neither of Gondor nor any other, great or small. But all worthy things that are in peril as the world now stands, those are my care. And for my part, I shall not wholly fail of my task, though Gondor should perish, if anything passes through this night that can still grow fair or bear fruit and flower again in days to come. For I also am a steward. Did you not know?’ And with that he turned and strode from the hall with Pippin running at his side.
Compare this with an excerpt from different fantasy novel, Joe Abercrombie’s Half the World:
She planted the sole of her boot on his arse, shoved him splashing down on his hands and knees in the latest wave, and on its hissing way out it caught his sword and washed it down the beach, left it mired among the weed.
She stepped close and Brand winced up at her, wet hair plastered to one side of his face and his teeth bloodied from the butt she gave him before. Maybe she should’ve felt sorry for him. But it had been a long time since Thorn could afford to feel sorry.
Instead she pressed her notched wooden blade into his neck and said, ‘Well?’
‘All right.’ He waved her weakly away, hardly able to get the breath to speak. ‘I’m done.’
I should stress that I am not meaning to imply that Abecrombie’s book is ‘wrong’, merely trying to show that a book’s chosen vocabulary has quite an impact on the way the story is perceived. Both Tolkien and Abercrombie are writing within the fantasy genre: Tolkien takes the creation of his world very seriously and is cautious about using no words which would suggest modern turn of phrase in the entire book, even when reporting the conversations of the hobbits, but more particularly in the ‘high feudal’ situations which occur frequently in the story as in the extract above. On the other hand, Abercrombie is keen to suggest a modern-day colloquialism, even using common slang words.
This only becomes a problem for a writer if the writer is trying to write in one style but inadvertently violating that style by using an inappropriate vocabulary. This creates a jarring effect upon the reader and signals an inferior work. Maybe this would be clearer if we ‘translated’ Gandalf’s speech into a common vernacular:
‘Unless the king comes again?’ Gandalf said. 'Well, Steward, it’s your job to make sure that there is some kingdom left until that happens, which only a few people now think will occur. You’ll have all the help you want. But I’ll say this: I don’t rule any realm, big or small, including Gondor. But all worthwhile things are in danger with the world right now, and I care for those. As far as I am concerned, I won’t completely fail in my job, even if Gondor dies, if anything survives that can still be beautiful or bear fruit and flower again in the future. For I’m a steward too. Didn’t you know?’ And with that he turned and strode from the hall with Pippin running at his side.
Choice of words here changes the tone and flavour of the whole interaction.
Range of vocabulary is just another instance of the writer as a point of control: select the right words, and the entire story can be coloured in a particular way. Choose inappropriate words, and the reader is jolted and can become confused.
Writers are in control, especially when they have command of the tools of their craft.