The Chicago Manual of Style is available online! Having been in the industry 20 years now, I’m thrilled about not having to pay big bucks for the latest edition every time a new one is released. Bookmarks, notations, related articles, etc.–so many perks right at your fingertips and easily searchable. Take advantage of their free trial and then spend only $35 per year for full access to the 2 most recent editions. Authors and editors should both be using this style guide for US style manuscripts. http://www.chicagomanualofstyle.org
For writing development this site is exceptional: http://thewritepractice.com/about/
Grammar Girl “Your friendly guide to the world of grammar, punctuation, usage, and fun developments in the English language.” ~ Mignon Fogarty
My favorite semi-colon comic strip: http://theoatmeal.com/comics/semicolon
Always make sure a service provider is not listed as DO NOT USE on this site: http://pred-ed.com Just because a provider isn’t listed as recommended, does not mean there is a reason not to work with them. That would be like saying just because any one site or media source doesn’t recommend your book, I shouldn’t read it. Also, FYI, updates are not getting done on this site, for at least the past year. I’ve personally received more than a dozen emails from clients who have sent recommendations for both of my businesses (Indie Author Publishing Services and The Pro Book Editor) and follow up emails to them asking why no updates were done. They are receiving no responses from pred-ed admin and no updates to include us on the list of recommended providers.
Another site to check for DO NOT USE providers: http://www.sfwa.org/other-resources/for-authors/writer-beware/