We Are Ciphers is a co-op letter-writing and code-breaking trpg for two players.
It's 11 pages, with straightforward but readable layout, and some useful diagrams.
The game itself is pretty structured, and it wants both players to learn, use, and decode actual ciphers. There's a narrative arc, colored but not significantly changed by your choices, and most of the fun is in codebreaking---supplemented by the meta-narrative that you and your partner create.
Overall, I think this is a fun game for pairs of people who really like puzzles, and who don't mind doing a little storytelling alongside them. It's well-written, and the rules are very clear, but it really does want you to learn ciphering. That's either a reason to impulse-buy the game, or a reason to pass it by, depending on how you feel about picking up the skill.