I own a book formatting company, and one thing I I've learned from it is that people from all walks of life, from physicians to politicians, all forget most of what they learned in school about proper writing by the time they're thirty. Most people don't even remember how to use something as simple as a comma, what the difference is between a comma and a semicolon, and when to use either. I've kept a BarCharts English Grammar & Punctuation Guide by my desk for the past 15 years. It's just a 4-page laminated pamphlet that will quickly tell 95% of us everything we need to know in order to write properly. For less than $7.00 it can't be beat. And being laminated, it will last a lifetime.
Even if all you write is an occasional blog post, this thing is a Godsend. Can't remember when to use an En dash or an Em dash? Don't remember when to place punctuation inside quote marks and parenthesis or outside them? How many dots go in an ellipsis? Do you ever capitalize the first word after a colon? If you place the title of a magazine in italics, then how do you set off the name of an individual article within that magazine? All the answers are just fingertips away.
I got my copy at a local B&N years ago. You can also find them online all over the place. Don't be without a copy. For $7.00 you can't go wrong.