It is only later on in the novel that Adso and William discover the secret behind the library and the labyrinth. They discover that the library and the labryinth are organised by the geographical origins of the books that are contained therein. The books are all grouped together according to where they come from, and the letters that they see result in the secret code that they can use to unlock the secret of the labyrinth and the library. Note what William says about this. As Adso says, "the plan of the library reproduces the map of the world." Note how William responds to him:
And the books are arranged according to the country of their origin, or the place where their authors were born...
The secret symbolism of the labyrinth and the library which William and Adso therefore subsequently realise is that it is literally a map of the world. Once they have realised this, they are able to use this knowledge to orientate themselves and find out where the secret room is that they seek.