I've had this book since childhood, and picked it up again today for a little jaunt down memory lane.
I remember being fascinated by The Mirrorstone, mostly because of its shiny 3D holograms. The illustrations are also wonderful, bringing the small cast to life. I couldn't recall much of the story, but having read through it again, I think that's because there isn't much of one! But the illustrations expand upon the fairly sparse text, and for a younger reader especially, provide plenty of fuel to the imagination, such that a book you can read in ten minutes fills hours of imagining.
I became familiar with Michael Palin later on in life thanks to Monty Python, and Alan Lee due to his illustrations based on Tolkien's Middle-earth. Having them (plus Richard Seymour) combine in this book is weird, but I think it is Lee's work that elevates this one. The fourth star of my rating is almost certainly based on nostalgia and the art work, but I stand by it. I suspect I will always have a soft spot in my heart for The Mirrorstone.