Maybe the "I Drew" clue is just telling us to look at the clocks - from the title of the first book - "The Secret of the Old Clock" (if memory serves - something like this happened in another puzzle - wasn't there but just reading up). The X's and Y's could be just confirmation we are on the right track. Or perhaps a clue as to how to interepret other clocks (???) as XX/Xy puzzles (???)
I wonder if there are mentions of clocks or, gulp, times for sure, on particular pages in the other Nancy Drew books that she gave Daniel. Maybe we (someone) needs to, gulp, read through the books. (I think she did that with the movies).
Anyways - off to work - happy reading
