Being conscientious about your spelling and grammar will help you become better focused on all the little details of your job. You don't need to pursue a degree to take a course that you think might better your communication skills at work. I suggest you call your local community college and talk with someone in the business or English departments and tell them exactly what you'd like to learn about. They can help you select the right course. Use the spell checker on your computer.
Most people spell check a document and then go on without thinking about it.
Rather than just fixing the errors and moving on, keep a journal of the words you misspelled and practice using those words in future documents. Although learning to spell correctly is a matter of knowing prefixes, suffixes and rules of grammar, practice and correction also play a part in tuning your skills. There are numerous books on how to improve your spelling. . . Books. . . You can also look up spelling games at Amazon or visit your local computer store.