David Crystal's top 10 books on the English language. (Via Books, Inq.)
Lynne Truss's ever popular book is, not surprisingly, not among them (and thank goodness).
My personal favourite, even though it's not entirely clear on the difference between syllepsis and zeugma, is Bryan Garner's A Dictionary of Modern American Usage (William Safire says, "Excellent."), though I suppose, technically, it's not on English, it's on American.
Honourable mention goes to Karen Elizabeth Gordon's The Deluxe Transitive Vampire: The Ultimate Handbook of Grammar for the Innocent, the Eager, and the Doomed (William Safire says, "A book to sink your fangs into."), for obvious reasons. A taste: agreement is demonstrated with such delicious examples as "An abundance of rumpled dahlias was deposited at death's door," and "The coven has voted to move its sabbath to a grove of olive trees.
Do you have a favourite book on the English language (or am I just weird)?