I don't have a copy of the text in front of me, so this is paraphrased. If someone could find the exact text, it might be helpful.
Partway through Twelve Months, Harry has a conversation with Mab in his apartment in the castle. In the midst of this conversation, Harry notes that Mab wanders over to one of his bookshelves and re-orders the books of the Chronicles of Narnia. Neither he nor Mab note this verbally, and to my recollection, Harry doesn't return to this point later in the book at all. What is going on here?
A few points to start:
1) I assume that Mab is adding information that was not previously present; that is, her actions are meaningful in that she is intending to communicate something. She is not randomly tidying Harry's bookshelf--that would be both out of character* and too trivial to mention as part of the narration, unless you want to postulate that this is intended as a meta-hint that something is wrong with Mab. I mention this last bit for completeness, not because I think it's plausible.
2) Narnia has two standard orderings: original publication order and internal chronological order. I would assume that the books started in one of these orderings (interestingly, both orderings are "official" in different contexts--you can find different editions with the spines of the books numbered in either ordering). If the new sequence contains extra information, the resulting order would have to be neither of these; I think going from one order to the other is too trivial to mean anything.
3) The clue could be a seven-digit number containing the digits 1-7 once each, as read from the spine of each book in the new order. Perhaps a phone number?
* In terms of Mab's character, I believe that her every action contains both intentional aid and the intentional infliction of pain. In this case, both aid and pain would be quite minor (an obscure hint vs. the irritation of a disordered bookshelf), but both would exist.
