I always read a book at Christmas, usually non-fiction, and for the one just gone I picked The Sense of Style by Steven Pinker
I first heard Pinker's name when I did a part-time course in teaching English as a foreign language at Manchester College of Arts and Technology twenty years ago. Along with Noam Chomsky and Stephen Krashen, he's a leading academic in the field of language acquisition theory.
The Sense of Style is a guide to English grammar and usage which, among other things, looks at commonly confused words ("disinterested"/"uninterested", "enormousness"/
"enormity"), the minefield of "lie, lay, laid, lain", the singular "they" for men and women, and whether to punctuate quotation marks the American way "like this," or the British way "like this". Pinker prefers the latter, dismissing the former as aesthetic fussiness by printers, and is pretty liberal when it comes to things most grammar experts frown upon (splitting infinitives and misusing "less"/"fewer"). It's a handy book for anyone who regularly attempts to turn out decent English prose, whether on a blog or elsewhere.