Probably the most important letter is a thank-you letter after an interview. This letter need only be two-three paragraphs and should be sent within five days after meeting with an employer. Failing to send a thank-you letter not only indicates a lack of courtesy, it can prove misfortunate if the employer is down to the final selection process. Also, never send a thank you card instead of a letter.
The first paragraph should let the interviewer know that you enjoyed meeting him/her, discussing the job and seeing the premises.
The 2nd paragraph (which may be optional) should let the interviewer know that you are still interested in the position. It would also be a place to mention anything that you had forgotten during the interview (i.e., a special skill that you have).
The closing paragraph should let the interviewer know that you want to hear from them again, and that you will make any allowances to come in for a second interview.
Remember that sending a letter is a form of courtesy to the employer and courtesy is always appreciated.
