Reference Website: Grammar Girl

One of the best "go-to" resources for when you have a question about English grammar, spelling, punctuation, or usage is Grammar Girl, a.k.a. Mignon Fogarty: QuickandDirtyTips.com. You can also follow her at Twitter, and if you like to listen to podcasts, she has a weekly podcast. She has also published some manuals and handbooks; here's a list of her Kindle ebooks. She has a fun and friendly style of writing, but she is also very detailed and informative: highly recommended!