Reddit mentions of Practical English Usage

Practical English Usage by Michael Swan is the most widely used grammar reference that I have seen.

On the "Elements of Style" by Strunk and White- this is not a grammar book, it is a style guide. That is to say, the intent of this book is to inform people who possess little to no training in document writing with standards in the hope that they will write better.

It should be noted that while William Strunk Jr. was a professor of English, E.B. White was not. White was an author of fiction most remembered for Charlotte's Web and Stuart Little.

Strunk first compiled a style guide for his own students at Cornell in 1918 (presumably in an attempt to head off grammatical errors and poor stylistic choices before they occurred, thereby saving him some headaches with constant error correction- a wise decision).

E.B. White happened to be a former student of Strunk, and after remembering the "little book" of his teacher, was tasked with revising it by his publisher 40 years later. He did, also adding a lot of material which was not in the original 40-odd pages.

E.B. White was not a grammarian, and some of the advice in "the Elements of Style" are just plain wrong (split infinitives, not using "which" to introduce a restrictive relative clause, beginning a sentence with "however"). As a guide to writing style for high-schoolers and some university students, this book is ok. As a definitive grammar book it falls decidedly short and actually does some harm (this book is the reason that many people refuse to say "than me" or "it was given to John and me").

This all discussed in more detail in an article called 50 Years of Stupid Grammar Advice by Geoffrey K. Pullum, a British-American linguist and Professor of General Linguistics at the University of Edinburgh.

Pullman is also co-author of The Cambridge Grammar of the English Language, which I haven't used personally but am sure is a good choice.