Codex Cipher - Concept needs help

[PoisonAlchemist]

I wanted to create a long term goal and mystery for my game so I came up with this basic idea.

There is a series of book called the Code Cipher, for lack of a better name. Each book is numbered on the spine and very very faintly enchanted to prevent wear and tear since they are very very old books. The interesting thing about them is that the numbers are not chronological like you would expect. When you have collected a large portion of the books together their minor enchantment begins to function on a larger scale and reveals the passcode or text for a Vignere cipher (extra points for the player who decodes it by hand).

My huge sticking points are this:
How are the numbers organized?
What is the passcode?
What is the message? Why is it important?

[Strolen]

Well, those are some pretty huge sticking points...

Number organization: That seems like it could depend on something world specific. Number of original cities. Number of original kings.  Number of xxxxx slayed by xxxxx that brought peace to xxxxxx.  They may not be incremental but they may still go in ascending order.  Each book may be based on one of these events so the contents and/or poem riddle in the book tells you what the number means.  Maybe on celestial bodies during the year. Each month the number changes but it might not be in any order. 2 Moons in January. 2 moons and 3 planets in Feb. 1 Moon in March. Something like that.  Maybe the ages that a series of kings died during a certain Age. Lots of options, hardest part is to come up with something the players would recognize.

Passcode: If you are just starting out there is no reason to decide now. For this and the message, since it won't come into play till further in the game, just leave it open and let the characters dictate it. It may change as you go depending on their reactions anyway. Just make sure the message is big enough to justify the buildup.

[PoisonAlchemist]

Thanks a lot, that was actually incredibly helpful.