Tone in business letters is critical in meeting your objectives. The Online Writing Lab at Perdue University has some great examples of how to get your tone right.
Make sure you have all the elements to write a strong letter.
This is the most common format. If you are unsure which format to use your best chance is to use block format.
The modified block, or indented, business letter format is less common, however, if your business uses this format use this example to guide your work.
Source: http://businessletterformat.org/
Should you include a cover letter? Yes. Let me ask it another way. Should you shake someone's hand when you first meet them? Yes. This document gives foundational and straight forward guidance for writing a cover letter.
This sample illustrates correct block letter format with letterhead and includes paragraph by paragraph instructions on content.
Source: Soma Jurgensen