Dear …
Asking you to write a __(character reference letter/letter of recommendation)__ for me was the best decision I’ve ever made. You really came through for me, __(name)__, and I just want you to know how thankful I am to you.
The letter you wrote on my behalf was absolutely wonderful. It means a lot to me that you would take the time out of your busy schedule to write up this splendid __(character reference letter/letter of recommendation)__ for me. I __(know that it will be very beneficial to me in my job search/am so thrilled that I got the (type) job …or… was accepted to…/etc.)__.
Once again, please accept my warmest thanks to you. I appreciate it so much.
Sincerely,