Before modern communications and speed of travel, letters of introduction (or recommendation) were a key piece of allowing utter strangers to meet and work together.
Basically, a go-between writes a letter affirming that the bearer of said letter is worthwhile and of the proper social standing to meet the recipient of the letter. To a degree, such things go on today, although typically turned into PDFs and emailed to recipients.
Statistically, they sometimes even work (I guess it depends on just who is writing the letter).
In the realm of DnD, a letter of introduction is a fine means for a newly-minted Wizard sort to find potential masters, especially if the game relies on PCs being properly trained to level up, not just earn enough xp and a nap. Presumably, said Wizard's teacher sends the neophyte off with a letter tucked into the starter spellbook, perhaps with a specific recipient in mind, perhaps not. Then off they go.
Given DnD and the apprentice-master relationship of wizards, such letters are excellent recipients of Secret Page (Illusionary Script if playing 5e) spells, allowing master wizards to pass on useful information without being obvious about it.
In addition to magic, more traditional ciphers and codes can be used on the letter. If the letter writer and recipient regularly correspond, there might even be nonchalant phrases that have other meanings, much like thieves cant, but for wizards.
Best of all is that such letters of introduction make excellent props for in-person games. Sealing wax and seals can be purchased (or done at home with a candle and something to press into the molten wax), and letters written on unlined paper, making for a tactile bit of gaming. So when the junior wizard shows up and approaches the PC wizard and hands her the letter, you can pass the letter to the player and see what happens.
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Of course, in a society where such letters of introduction are common among the nobility and the magic-users, there are bound to be folks wanting to take advantage of this. Such letters allow rogueish sorts to practice subterfuge, forgery, and the like, presuming they can lay hands on authentic signets.
Perhaps the best example of rewriting such a letter can be found in Shakespeare's Hamlet, when loyal-but-not-bright minions Rosencrantz and Guildenstern carry a letter that Hamlet rewrites to his advantage. Hamlet escapes and R and G are executed by the letter's recipient - literacy and forgery FTW!
Such letters and signets can also lead to false orders, bills of sale, and blackmail for the workaday rogue, and if magic is involved, utter mayhem for the letter reader (perhaps this is what curse scrolls really are). With the various word-based spelltraps available, it makes things quite... froggy.
Designated readers, perhaps.
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Letters of Introduction make for interesting and different treasures, maybe not so much discovered as awarded. A letter that gains its bearer an audience with the King's Court Wizard or even the King's Major Domo can be quite useful, especially for those of a political bent. For a society built around such methods, rubbing shoulders with movers and shakers is important, and letters allow just that option
All that said, with commonplace letters of introduction comes a presumed level of literacy that doesn't jive with history, nor frankly, my homebrew game world. Wizards and most Clerics are certainly literate and begin the game that way. Other PC classes might be, depending on social class, so probably nobles, but not all of them. Everyone can learn to read (Intelligence score depending, of course), but terribly few start out with it.
Except elves. Elves can do everything, because they are immortal.
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Some related magic items:
Sealing Wax of Imperviousness - letters sealed with this wax are impervious to weather, fire, water, bugs, and time - the sheet(s) of the letter and the ink upon and within the letter remains clear and legible until the seal is broken, and then it reverts to normal ink on vellum or papyrus or parchment.
Ink of Compulsion - letters written in this ink read like other letters, with the exception that the reader believes whatever is written in the ink, no matter how unlikely it is. It requires substantial proof (or a dispel magic or curse) to make the reader believe otherwise.
Forger's Signet - This signet has a blank face, but when pressed against a complete seal, it shifts and adapts to match the signet; after one use (or an hour's time), the face goes blank again and the Signet needs to rest for a full day. These masterwork pieces of the forger's craft tend to get owners killed in painful and permanent fashion.
Animated Signet - the name is misleading, as the signet itself isn't animate, only the image pressed into the wax. As might be imagined, forging these signets is damn near impossible without powerful magic, the use of which gives itself away.
Parchment of Masking - this parchment is specially treated to allow for a secret message to be hidden, as per the secret page (or illusory script) spell.
Ribbon of Concealment - a letter wrapped in this ribbon, and then sealed with wax, cannot be seen or found except by the sealer, the designated courier, and the designated recipient. As might be imagined, this is incredibly useful for delivering secret messages between rulers or lovers. Or both.
Speaking of wax and ribbon...