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!