Letter of inquiry

A Letter of Inquiry can help you uncover the hidden job market. Sometimes these cover letters are called marketing letters.
A letter of inquiry should be followed up with a phone call as this will increase your chance of getting your "foot in the door". Hopefully such a phone call can lead to a meeting or interview.
Good Letters of inquiry
Inform the reader why you are writing within the first sentence.
Read the how to write a cover letter guide, most of the tips are the same for letters of inquiry.
Have a look at job application cover letter article, it offers a lot of insightful tips on writing job related correspondence.
Highlight creative ideas to generate interest.
In all cases make sure you follow the Guide on writing Job related Correspondence which gives a lot of specific tips on writing different types of Cover Letters