ImaniOU recommended Michael Swan’s “Practical English Usage”. It’s available from Caves now. I don’t think they sold it before, at least not in Taichung.
This is a really useful book. It contains guidance on many of the usage questions that come up. It uses statistical analysis to find out how people actually use the language, but it also tells you if a usage is considered incorrect according to prescriptive rules.
It has information on register, on differences between written and spoken English, and on the differences between Brit. and Am. Eng. usage.
Caves is selling the third edition which has more examples drawn from corpora, more information about writing/speech usage differences, and is overall an even heftier book than the last edition. Not sure about the price but I imagine it’s somewhere around the 1000NT mark, which may sound a lot but it’s worth it for any serious teacher.