As someone whose business it is to know (and recognise, and apply) the differences between US, UK and Australian English, I loved the ‘Shop Talk’ Q&A on the Chicago Manual of Style Online.
Most people know that UK (and Australian) English use –our and –ise endings rather than the –ize and –or of US English*, but seldom appreciate (in both senses of the word**) the vague and nebulous differences that are so subtle and instinctive they’re almost impossible to list or categorise.
This particular excerpt is very close to my heart:
…the word cookie is more and more common in the UK, but it would never refer to the hard, sweet biscuits that the British dunk in their tea (and Americans would call cookies). Cookie is used in the UK just for the kinds of cookies that have come over from America—the big, soft kind that are sold individually in shopping centers and coffee shops.
For almost infinitely more on the topic from Lynne Murphy, read her blog, Separated By a Common Language.
Bill Bryson’s book The Mother Tongue: English and How it Got That Way is also heavenly. I suddenly have an urge to reread it. Again.
* Not to mention colorize/colourise.