Here are some sites I turn to for advice and guidance.
After Deadline, New York Times blog on usage and style
Copyeditors’ Knowledge Base, by Katharine O’Moore-Klopf, an experienced editor, particularly in life sciences
Language Log, a blog about language by linguists
Lingua Franca, a blog about language by various authors (some editors, some linguists, all smart)
An American Editor, by editor Rich Aden
Grammar Underground, by June Casagrande
Grammar Girl, Mignon Fogerty (and others) on grammar, usage, and mechanics
Guide to Grammar and Style, by Professor Jack Lynch
The Blue Book of Grammar, by the late Jane Straus
Guide to Grammar and Usage, by the late Charles Darling, maintained by Capital Community College
Here are some books I use:
Garner’s Modern American Usage, Third Edition, by Bryan A. Garner, Oxford University Press, 2009