I do a lot of writing for other people and though I have many of the grammar rules sorted, like when to use 'which' and 'that,' I still have my grammar books close at hand to double check on some of the more complex rules. Call me old fashioned, but I still love the touch and feel of paper books."Collins: Improve your Grammar,' 'Booher's Rules of Business Grammar' and Strunk and White's 'The Elements of Style' are my constant companions, oh, and of course we can't forget Google.
You can search any question you may have about grammar in Google and you will find an answer to it. Some sites are a bit complex, but some are great at explaining the grammar rules in a simple, easy-to-understand way, which is important because English grammar is not always that simple. Even the words used to talk about grammar sound like the gobbledygook. I wage warfare against it everyday in my writing; past participles, subjunctive mood, nominative pronoun. Who invented this stuff anyway? No wonder we need help!
There are also grammar checkers where you can insert your text and it will return results for any errors you have. Try out http://www.grammarly.com/?utm_expid=23712475-3 There are numerous free downloads for grammar checks and more advanced ones you can pay for.
Sites like http://www.grammarbook.com/ or http://grammar.about.com/od/punctuationandmechanics/a/punctrules.htm can be useful, but there are lots more you can check out.
Be mindful though, punctuation and spelling can differ between American English and UK English so make sure you use the version that's right for your audience.