Over the course of the series, a few letters have been written.
He wrote it after Bonnie magically linked him and Elena so that she could live after Klaus' sacrifice ritual and it gave her a chance to not come back as a vampire. However, for the spell to work and for Elena to live, John had to die. Knowing that he would never talk to Elena again, he wrote the letter and gave it to Jeremy to pass along to Elena, including his ring.
Today I did things I abhor to protect the one thing I value most, my family. If anyone can understand it, it's you. Your compassion is a gift, Elena. Carry it with you, as I will carry my regret. Always and forever.
If anything happens to me, carry on my work. You have the other ring, let the ring make you strong. The council must be cleansed before the work can begin.
Pastor's Letter was written by Pastor Young to his daughter before he killed himself. Connor found the letter in his house and took it with him. Thus, April never got the chance to read the last words of her father.
I'm sorry for what you're going through. What I did was a necessary sacrifice, and I'm afraid it's only the first of many more sacrifices to come. There's always been an evil that spread through Mystic Falls, but now a greater one is coming. My death is but the first in the war ahead. I will see you again when we all find our salvation.
Until then, I love you.
Dear Car, I miss you more than I can put into words. But I know as long as Klaus is alive, I can't come home. He won't stop trying to kill me. And you won't stop trying to protect me. Which means the only way to make sure you're safe is for me to go and never come back. I left Matt the deed to the house. It will protect you in ways that I can't anymore. I will always love you.
I hear Elijah has refused you the cure, and in return you have refused my freedom. Shame on you both. But while you boys sort our your problems, I have one last thing to offer you. I have caught wind that there is a witch in New Orleans named Jane-Anne Deveraux plotting a move against you. Hunt her down. What she has to say will rattle you so deeply to your core that chasing little old me will be the least of your concerns. It's been a fun five centuries Klaus, but I've worn down too many good heels running from you.
Love, and hate.
Sorry I had to kill your boyfriend, but it was the motherly thing to do. Suicide, however, not very motherly of me. I ran from my enemies for 500 years, and then one day, I stopped. Now a new enemy wants me dead. Sure I could run from the Travelers, but there's still one enemy that I can never escape, time. So call it pride or vanity, but after the life that I've led, I refuse to drift off into the sunset.
Hayley's LetterHayley's Letter was written by Hayley to her unborn daughter Hope Mikaelson in a flashback in From a Cradle to a Grave. She wrote her in her daughter nursery and Klaus asked to her if it was a love letter destined to Jackson, Elijah or himself. Later, when she decided to give her newborn to Rebekah, Hayley put her letter in her daughter's bags.
Dear Zoe..or Kaitlyn... or Angela. To My Little Girl,
Your dad just asked if this was a love letter. I guess it kind of is. 'I never got to know my mother. I have no idea what she must have felt when she carried me. So I thought I’d write to you, so that you could know how happy I am at this very moment. How much your father and I can’t wait to meet you. And I want to make you a promise, three things you will have that I never did. A safe home. Someone to tell you that they love you every single day. And someone to fight for you, no matter what. In other words, a family. So there you go, baby girl. The rest we’re gonna have to figure out together.
I love you, Your Mom.
Elena's LetterElena's Letter was written by Elena to remind herself that she loved Damon. She read it in "Black Hole Sun".
If you're reading this, then somebody spilled the beans about your selective memory loss (my money's on Caroline). Yes, you loved Damon. You loved him with a passion that consumed you, and then when Damon died, the void you felt was too deep, too dark. Facing an eternity without your soulmate, you went off the deep end. You turned into someone that you weren't -- a monster. Alaric can restore your lost memories, all you have to do is ask, but I hope that you don't. I tried it the other way and I didn't see an end to the pain. I want you to rediscover yourself in the absence of the one who defined you. If you feel any hope for the future at all, then you're already better off. You've been given a chance to start over...I want you to take it, I want you to be happy.