There are plenty of books on improving your writing, and here’s one that works immediately. It’s more direct than a thesaurus and is instantly applicable for deepening the breadth of your vocabulary in the moment, while you are writing. Whatever word you’re thinking of using, check it out in the ol’ Synonym Finder and you’ll likely find a better one — or at least get to thinking about other possibilities.
Since we all write with our own voices, it’s sometimes helpful to have a tool at the ready to infuse some color when called for, especially in moments of creative befuddlement. I borrowed this copy from my father before leaving for college, and reference it to this day. It’s definitely gotten use, hence the duct tape keeping the cover attached to the spine.