As of October 2015, some books about English grammar with high ratings on GoodReads.com include "Understanding and Using English Grammar" by Betty Schrampfer Azar, "Practical English Usage" by Michael Swan and "Essential Grammar in Use" by Raymond Murphy. Other books on the topic are "The Blue Book of Grammar and Punctuation" by Jane Straus and Lester Kaufman, "Our Mother Tongue: An Introductory Guide to English Grammar" by Nancy Wilson and "A Guide to English Grammar" by Bessie Brooks.
Free online guides to English grammar include the Capital Community College "Guide to Grammar and Writing", Purdue University's "Online Writing Lab" (or "OWL") and the "Grammar Handbook" from The University of Illinois.
A style guide provides consistent guidelines of English grammar usage and composition for writers and journalists. English grammar varies slightly from one English-speaking country to another, so style guides are often specific to that country. In the United States, the most widely used style guides include "The Associated Press Stylebook," "The Chicago Manual of Style" and "The Elements of Style" by William Strunk Jr.
Some books about English grammar are written to be approachable and easy-to-read. Examples include "Eats Shoots and Leaves" by Lynne Truss and "Woe is I: The Grammarphobe's Guide to Better English in Plain English" by Patricia T. O'Conner.