I own exactly one book on the craft of writing: a mass market paperback edition of Stephen King's On Writing. The first half of the book has some autobiographical stuff, which is interesting, but what you really want it for is the second half. King calls it the "Toolbox" and it is basically a course in creative writing. He uses the toolbox metaphor to describe what skills writers need. My copy is full of turned-down page corners and underlined passages, and I find it helpful to read through it from time to time.
The first section is about vocabulary. The sentence I have underlined here is this: Remember that the basic rule of vocabulary is use the first word that comes to your mind, if it is appropriate and colorful. King includes several examples by different authors, some that used words most of us would have to look up in a dictionary, some that used more basic vocabulary. He's not advocating that you dumb down your work, but I do think he makes a very good point about clarity. Think for a moment about your own reading experience. How did you feel about a book if you had to constantly stop to look up a word, or figure out how to pronounce something, or read a sentence more than once to figure out what the author was trying to say? That kind of writing pulls me out of the story and makes me take note of the author, rather than stay engrossed in the story. I don't read a book looking to be impressed with an author's vocabulary, obscure metaphors, or purple prose. I want to get lost in a good story, and it's hard to do that if the author insists on getting in the way.
Word choice is incredibly important. If you have any doubt, pay attention to current events for about a week and see how many times a language-challenged politician has to walk back some dumb statement they made and issue a "clarification." Novelists don't get to issue clarifications, we have to get it right the first time. There's no telling how much time I've spent staring at the screen, practically begging the universe to send me the exact right word. And when your editor has pointed out you've used the same word three times in the same paragraph and need to come up with two substitutions it can be tricky. I've gotten help from using a thesaurus, had to completely re-write passages, and sometimes just had to beat my head against a wall for awhile until the words came to me. Struggling over word choice can be frustrating but often when you get it right, you'll know it. You'll be able to read over the sentence or paragraph you've been working on and know you got it right. No clarification needed.