This is the Hundred-and-tenth in my one-book-at-a-time bookshelf.
One of the curiosities of memory is that I recognise the cover of this particular edition from my childhood. It has a familiarity that invokes a sense of the book and the long ago time when I first read it. Checking the publication date confirms my memory. Paperbacks were such a great invention!
While secret passages are to be expected this book takes them to a new, inspirational level! There is also a darker side to the villains in this book that moves it a notch beyond the previous books in the series.