Many students I have taught over the years have said to me that they only want to improve their speaking skills, and they are only interested in having conversational classes. While speaking is obviously essential to any language, (and conversational classes are by far my favorite),it will only take you so far.
If you want to be taken seriously by English native speakers, especially in a business environment, you will have to learn how to write well (yes, spelling included!). Being able to spell correctly and form coherent, well structured sentences instantly evokes respect.
You can only truly say that you have mastered a language if you can listen to, read, speak AND write it. And as with any kind of learning, the one aspect complements the other. So in order to write well, you must read! While grammar exercises and memorizing vocabulary lists are very helpful, reading is by far the best way to learn to write. It lays down patterns in your brain for you to recall and mimic in the future. The more you read, the more naturally you will become able to construct sentences yourself.
So remember, don't just speak, speak, speak... write, write, write!