One of the hardest things about business English is that it has its own set of vocabulary and conventions. Even if you have good general English often you need to boost the vocabulary that you know. A great self-study book called 'Business English in Use' written by Bill Mascull and published Cambridge University Press can really help to boost your learning. It comes in various levels such as elementary, intermediate and advanced. It has approximately 66 mini two paged chapters on a wide variety of subjects. Each chapter has a detailed explanation and then exercises for you to work through. It is available in most major English selling book stores in Turkey, including Nezhi and D&R. It also importantly has an answer key so that you can see how you are progressing through each chapter. This resource is definitely worth considering, in order to supplement your business English study.