My accent reduction students usually have an advanced knowledge of English, but they still occassionally make mistakes with English grammar or vocabulary usage. Here are some books that I recommend that they purchase to perfect their skills. I have also included the links to Amazon.com where you can purchase the books. I have used these books when teaching college ESL classes as well as with my private clients who need to brush up on their skills.
"Understanding and Using English Grammar" by Betty Azar. This is an excellent grammar book. I have used it in my classes for many years. Very detailed.
The Article Book by Tom Cole and Three Little Words by Elizabeth Claire. I recommend buying these together. If your native language does not have articles (a, an, the) these two books will be helpful to you. They are pretty short but they cover a lot of the rules and exceptions of article usage.
Speak Business English Like an American by Amy Gillett. This book has very useful idioms and expressions that are used in the workplace. Great self-study and audio.
