Wikihow is a very interesting attractive website to obtain a guideline on how to write the following types of letters:
->LETTER OF RECOMMENDATION
If you want to have a look at some interesting expressions for this type of correspondence click here.
-> LETTER OF COMPLAINT
Useful expressions to complain? Try this link and this pdf.
-> LETTER OF APPLICATION
A useful link about helpful vocabulary for these is available here. Do you want to test yourself online about this kind of letters? Try this test from the BBC.
-> LETTERS OF REQUEST
How to write a request letter
Sample request letters