The Purdue Online Writing Lab, from Purdue University, is a good source for information about different aspects of business communication, as well as other types of writing and citing information. For business communication, see specifically:
Although your texts provide the best information about writing business proposals, there are also some other library resources that can help:
Annual Reports provide many examples of corporate communication. If yo use the annualreports.com link below, you will get primarily the glossy annuala report provided to shareholders. In that report, look for the "letter to the shareholders" - a communication from the CEO of the company that serves as an executive summary of the companies results for the previous ryear.
If you use Rank and Filed, you may be able to access the glossy annual report, but more likely, you'll find the 10-K which is the annual report provided by the company to the Securities and Exchange Commission. When looking at the 10-K focus on Part I - the description of business. This is a summary of what the company does from the perspective of the company.