I really recommend "Oxford English: a guide to the language" by Ian Dear. It is/was published by Oxford University Press but if you look around on eBay, Abebooks, amazon secondhand etc., you can often find a very cheap "IBM Edition" -- they co-published it along with a dictionary of quotations (presumably they helped compile the latter).

It's a really fantastic volume that goes into such detail about Indian Standard English, for example, that it will make you love English even more.