With the peculiar balance of humans involved (it's notable just how little the human children, who you might think would be the easy way in to the story for the target audience, have to do and say) and the increase in the bizarre, we're on very different ground. In the end, as well, there is almost a pastiche side to things, which brings a greater edge to proceedings, serving to make the book more amusing perhaps for the curious older reader.
Not too old, I would suggest, for there is a side to the author's style which will exclude the more literate and discerning. Nobody says anything - instead they vocalise in the most dramatic way possible. The bickering magpies aren't that great fun, even when using a book of spells, and the human/cat relationship is almost cloying in its use of hugging and complete understanding. But I do think this series has some oomph and panache to it. The second book, concerning the Crown Jewels and the Tower of London, was but a stepping-stone to ancient Egypt, it seems - who knows where Atticus might end up in the future? These are books designed purely for entertainment, and at that they're very successful, so I assume we will get as many as Ms Gray can find rhyming titles for.