[unethical_ban]

The first sentence is about the writer; the second about the reader.

If writers aren't learning new words, then readers can't learn from them. When do writers learn new words? From reading! Who writes what the writer reads? Writers.

King seems to be saying "the best way to discover new words is through the labor and chance of picking up the right books and finding some words you had not read before". So all words that can ever be useful have already been written or will be invented by fiction writers, and it is up to you to read a variety of styles and types of fiction rather than the compendium on your shelf. I find this notion silly.

The answer of course is a blend. If someone is leaning on a thesaurus to make bad writing good, there will be a problem, too.

[mlyle]

There's a huge potential collection of words you can use. But there's also the words in the current zeitgeist that will be meaningful to your readers.

The only way you can learn these words and give them a proper weighting of "usefulness" and understand their fundamental nature is to read. Yes, you can get definitions and similar words from tomes, and even examples of usage, but you don't really understand the connotations or colloquial uses until you have seen other people use them in full context.

Of course, there are times when one might want to dredge up an infrequently used word, coin a new one, or transport something from current spoken vernacular onto the page. This should be a tiny fraction of where our chosen words come from, though. Reading is the tool to build usable vocabulary, even if it isn't fetch.