Trying to identify a sci-fi novel for my dad. It was a book he borrowed from me in or before 2003:
The main plot point that we both recall is the sudden appearance at four distinct points of the Earth's surface of pyramid structures which seemed to have simply pushed their way through the surface of the Earth. The main character eventually came to the conclusion that
these pyramids were, in fact the corner points of one enormous pyramid, the main body of which lay within the body of Earth itself.
My initial thoughts were that it was 'Sphere' by Michael Crichton or 'Fade Out' by Patrick Tilley. But after finding several plot summaries of both around the web I don't think it was either of them. In 'Fade Out' the pyramid (according to what I found on the web) grew from the ship, and in 'Sphere' I don't think there were any pyramids.