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