Please keep the letters between 500-1000 words. There may be some exceptions, such as if a child is writing the letter, or if your writer is particularly verbose. Generally, this range is good enough to introduce the relevant details of your world(s), the nature of the conflict and possibly the consequences and urgency of that conflict.
Personality
Feel free to add some personality in the actual format of your letter. If your character is a formal diplomat, a business-like communique is fine. If your character is a brutish barbarian, ignore all pretense of grammar, spelling and formality.
Mature Content
I'd like to keep these letters appropriate for all ages. You can introduce some mature themes and subjects, just make sure you're not getting too graphic. "Locust and Spider" by Virginia Dare is probably the upper limit of the scary-scary. The later Harry Potter books are a good example of straddling the fence between young and mature audiences.
Legalese
Just to keep things open and simple, let's say that when you submit a letter, you retain all rights to your work. You can use them for whatever, anywhere, any time. if your letter is particularly cool, I might ask you if I can include it in the book. We can discuss further details if that comes up.
Deadline
If you want your letter to be included in the book, I'm hoping to have it ready to purchase by August '08, so there's plenty of time to write one or three or ten. ^_^ I have a ridiculous goal of having 100 letters. If you don't write 'em. I will. Please, make my job easy! :D
Genre Restrictions
I would like to use the premise of the "small worlds" as an excuse to produce some crazy settings ranging from fantasy to scifi. The only potential obstacle if your letter features "high magic" or massive cosmic powers. Just remember, even if your letter-writer does have crazy powerz, he or she is still asking for help, so there's presumably a problem that even those vast abilities can't handle. It's like when Superman asks Batman for help or when Aesop's lion needs a mouse to remove a thorn from his paw.
The pilgrims have a nominally useful "magic" power, the ability to fly. Still, that's a very rare ability and not one that can be used without extensive training. Consider it analogous to the Force wielded by the Jedi and the Sith in Star Wars.
Otherwise, go crazy. Steampunk? Sure. Airships? Yeah, totally cool. Dragons n' stuff? Alright! Feel free to introduce weird stuff like spirits or talking animals, but the core premise remains the same:
Someone needs help, putting the player-characters firmly in the protagonist role.