Skyrim:A Lovely Letter

Quick Walkthrough
# (Optional) Learn about the love triangle from town people. 
# Talk to Sven or Faendal about the love triangle.
#* (Optional) Talk to the other man involved in the love triangle (Sven if you talked to Faendal first and vice versa).
# Give the letter to Camilla Valerius.
# Return to the man you decided to help to finish the quest.

Detailed Walkthrough
The Triangle
When you visit the small town of Riverwood, the local beauty, Camilla Valerius, has sparked the interest of two bachelors in town. The local bard, Sven, and the wood elf hunter Faendal, are both vying for Camilla's favor; and will resort to slander and libel to win it.
What a Tangled Web We Weave
If you approach Sven first, at the Sleeping Giant Inn, you find out about his love for Camilla, and about his suspicions about Faendal's involvement in her life. Sven will then ask you for a favor: deliver a fake letter supposedly written by Faendal to Camilla, seemingly breaking things off with her due to her being an Imperial, and he a true-born elf. 
Or you may speak to Faendal first. He can be found near the lumber mill chopping wood or wandering the village. Faendal also has feelings for Camilla and a desire to see Sven out of the picture. He asks you to deliver his own fake letter designed to disrupt Sven's wooing by outlining out the household chores he has planned for Camilla. 
With either letter, you can go straight to Camilla, or show the supposed letter to the other man prompting them to hand over their own forged letter for you to deliver.
When We Practice to Deceive
When you decide who you want to help, either Sven or Faendal, deliver the letter to Camilla.
Camilla is usually at the Riverwood Trader, but at night she can sometimes be found at the Sleeping Giant Inn.
When you talk to Camilla, you have two ways of proceeding. You can lie to her and give her the forged letter as is, or tell her the truth about the ruse. In either case, she will effectively break off her interactions with the loser. Speak with the winner to collect your reward.

Notes
Faendal makes a good follower because he is an Archery trainer. You can train the skill and later retrieve all the money you spent on the training from his inventory.
 This exploit is no longer possible as of version 1.2.6 of the Unofficial Skyrim Patch.
It is not possible to complete this quest in a manner that has both Sven and Faendal as followers and to have their dispositions friendly towards you.
In a particularly sneaky move, you can marry Camilla yourself after completing The Golden Claw quest. Strangely enough, marrying her does not solve this quest. Solving this quest after marrying Camilla will cause the 'winner' to appear at your house, should Camilla have moved in.
It is possible to turn in both fake letters to Camilla and tell her that both Sven and Faendal tried to manipulate her. Go to either Sven or Faendal, then take their fake letter to the other NPC, and when he gives you their own fake letter, pickpocket the second fake letter from them. You can then go to Camilla and give her both of the letters. Whenever you decide to tell her the truth or not, both Sven and Faendal will act more hostile to you, greeting you with a phrase such as "You're nothing but trouble." However, talking to either one of them to conclude the quest will change their dispositions as normal, respectively.
If you turn in both letters to Camilla, the first of her suitors that you talk to will pay you and be available as a follower. The second you talk to will not follow you afterward. They both may say positive things. Sometimes one says negative things.

Quest Stages