Today, I am offering a quick word of advice. Go to Amazon or to your local used book store. Carefully locate the reference materials and then studiously go over their inventory. If they have a dictionary that is over 50 years old, buy it. The bigger it is, the better.
This is going to augment your more modern dictionary with an endless number of words they no longer list so there is room for things like “YOLO” and other slang. (Could you hear my sarcasm there?) This can prove to be an invaluable resource for your writing. If you are writing something set in a particular time period, finding a dictionary dated from just a few years after that time can be incredibly helpful in determining what words weren’t around then.
As for the Thesaurus, avoid having them like the plague unless you particularly need a character to sound pretentious. If you don’t know another word for what you want to say as a writer, the odds are that your reader won’t know the word you find either. Alternatively, they will know the word and it will turn out you used it wrong somehow.