What I consider to be the most crucial resource is The Chicago Manual of Style. Any question about grammar, citations, editorial styles and more, this is the go-to resource for writers. I own a hardcopy, but you may subscribe to the website to access content. Go to the site and give the free trial a try.
I also keep Grammar Girl’s Quick and Dirty Tips handy. This is a good reference for those moments when you just need to find a general rule or to figure out whether to use “between” or “among”, the book and website (and app) can all be very handy.
If you have any question, use a reference. Don’t guess. If you’re wrong and use the wrong “your”, you may lose a client and damage your reputation.
Keep on writing… and writing well.