Apocalypse World was one of the first RPG systems that was opinionated, that is, the designer had an opinion on the 'proper' way to play, instead of just handing over the rules. This translated through in the AW Agendas and Principles:
Agendas
Make Apocalypse World seem real.
Make the players’ characters’ lives not boring.
Play to find out what happens.
Principles
Barf forth apocalyptica.
Address yourself to the characters, not the players.
Make your move, but misdirect.
Make your move, but never speak its name.
Look through crosshairs.
Name everyone, make everyone human.
Ask provocative questions and build on the answers.
Respond with fuckery and intermittent rewards.
Be a fan of the players’ characters.
Think offscreen too.
Sometimes, disclaim decision-making
These will be useful context for the Love Letters concept, which was introduced in the optional rules section of AW (called 'Advanced Fuckery' instead of Optional Rules, natch). In the AW book, they're not even called Love Letters, just presented as custom moves for "Catching Up, Setting Up", all opening with "Dear Hardholder" (Hardholder is one of the playbooks, so think of it as the D&D Class or Numenera Type), then presenting something that occurred between sessions of play as a Move. For those who don't know, Moves are the basic bit of Apocalypse World system mechanic, where you roll + a Stat, on a 10+ its a success, 7-9 is a success at a price or with a complication and 6 or lower is a failure. And the letter closed "Love and Kisses, Your MC" (MC=Master of Ceremonies, the AW GM title).
You can see some good Apocalypse World Love Letters written up at
No Ordinary Obsession, if you need some examples to peer at. What I want to talk about is why (and how) to use them in Numenera (or any other game really).