You can use the Emailer to do this!
Set up the letter you wish to send - complete with tokens for personalization. You can even use the "Preview" button at the top to preview what the letter will look like for each person with the tokens filled in.
Then, instead of hitting the "Send" button, click on the "Print" button at the top of the Emailer pane and you will have the option to print out the letter for each volunteer.
Combined with the Mailing Labels pane in MSP, you can easily send out a personalized letter to everyone!