Interestingly this book does not give an answer to the quesiton posed, so do not be disillusioned when you learn that
It is more about the interesting facts of its physics, geography and dimensions, as well as "the ringing sound" that suggests it may be hollow.
It makes for a very interesting read, one from which you can draw some of your own conclusions as to its origins. Some of the conclusions/beliefs the writers have are questionable, and open to personal interpretation, but on the whole it does beg the big question, as the geometry given in the book is actual verifiable scientific/mathematic FACT, what are the chances it is a 100% natural object?