Boss Praises My Job!

I am so happy today, because my boss praise on my job this morning. She told me that the letter I write to my supplier presented very good company image, because of I'm using polite and formal English. This was the first time someone praises on my English! I know my English are not good and I am trying my best to improve it. I even told my boss that I am searching for English e-learning course to help me improve my grammar. However, I haven't found it yet. Actually, there are so many website offering English Grammar or Writing skill online course, I just do not know which one is better.