Lauren Kate (author of the Fallen series) is a very talented author. She is currently working on another book, but would be fine if you sent her a letter of your appreciatingness or just a fan letter of how much you like her books.
This is the address to send the letters to:
PO Box 461514
Los Angeles, CA 90046
Or if you prefer e-mails, you can get that from her website http://www.laurenkate.com/