The Best Punctuation Book, Period. by June Casagrande is an excellent reference that lives up to its title.
The Blue Book of Grammar and Punctuation by Jane Straus, Lester Kaufman, and Tom Stern is a breezy and practical guide to keep by your side.
The Chicago Guide to Grammar, Usage, and Punctuation by Bryan A. Garner provides one of the most thorough, comprehensive explanations of the parts of speech I have read.
The Classic Guide to Better Writing by Rudolf Flesch and A.H. Lass is an old treasure I always had but only recently discovered.
The Elements of Style by William Struck Jr.and E.B. White is still a good reference for clear, no-nonsense writing.
HBR Guide to Better Business Writing by Bryan A. Garner is brief and helpful for daily writing in your job.
Mortal Syntax: 101 Language Choices That Will Get You Clobbered by the Grammar Snobs—Even If You’re Right by June Casagrande is an informative read on the clobbered usage of grammar.
Nicely Said by Nicole Fenton and Kate Kiefer Lee is a required reading for anyone who writes content on the web.
On Writing Well by William Zinsser reminds me the reason I have been blogging for over ten years.
Origins of the Specious: Myths and Misconceptions of the English Language by Patricia T. O’Conner and Stewart Kellerman debunks words and usage that are often misunderstood.
Woe is I: The Grammarphobe’s Guide to Better English in Plain English by Patricia T. O’Conner makes grammar refreshingly clear and enjoyable.