This page will change from time to time. Check back.
www.apstylebook.com/ — This is a gold standard for journalists. While I like flipping through the spiral-bound print version, there is a more traditional paperback style and it’s available online. It’s inexpensive and useful for non-fiction writers, since so many places use it as their default stylebook.
Chicago Manual of Style
www.chicagomanualofstyle.org/home.html — This is a gold standard for academic writers. If you don’t want to buy it, look for it in a library near you.
owl.english.purdue.edu/owl/ — This resource has great tips on U.S.-style grammar and usage. (There are no Oxford commas here.) It’s free and available 24/7. It is designed for academics, but any writer can use it.
www.poynter.org/ — For non-fiction writers, this is a great site not just for writing tips, but also for industry chatter and for ideas that might turn into your next book.