It is definitely described as a labyrinth in the text itself and numerous characters refer to it as a labyrinth. Note the way in which the library is secret, and that only certain people are allowed to go into it, and only certain people know of its layout and the ways in which books are organised and positioned. This is something that Jorge uses to his own benefit, as he, although he is literally blind, can "see" better than other characters in the library because of his knowledge of its layrinthine paths and its darkness. What is far more interesting and disturbing about the library however is not its presentation as an impenetrable labryinth without a seeming pattern or order, but the way in which it is presented as being a very gothic and almost supernatural setting. Note what Adso says about the books in the library in the following quotation:
Until then I had thought each book spoke of the things, human or divine, that lie outside books. Now I realized that not infrequently books speak of books: it is as if they spoke among themselves. In the light of this reflection, the library seemed all the more disturbing to me. It was then the place of a long, centuries-old murmuring, an imperceptible dialogue between one parchment and another, a living thing, a receptacle of powers not to be ruled by a human mind, a treasure of secrets emanated by many minds, surviving the death of those who had produced them or had been their conveyors.
The library therefore is presented as almost a supernatural force in itself, a place of danger and of challenge, in which characters stray at their peril. The wya in which "books speak of books" highlights a very important theme to Eco as he explores the inter-relations that one text has with another. It is therefore extremely fitting that the secret library is laid out as if it were a labyrinth, because it reinforces the idea of the books representing an accumulated knowledge that is difficult to penetrate and hard to grasp, just as the labyrinth itself is very challenging to solve.