This is really two great books. The first is a wizard's experiment gone wrong at the Unseen University. A chain reaction in their squash court (sound familiar?) has released an unprecedented amount of thaumic energy. Before it could be channeled safely, it materializes a world, in fact a whole star system. But this world isn't a disc, it's round --
The second book is a witty, well informed scientific commentary on many things, but especially on the history of life on earth. (I only noticed one mis-step in the real science, a statement about the stability of a an oxygen isotope. They probably slid that error in to make nitpickers like me feel smug.)
The problem is, this is just one book. Chapters alternate in odd-even pairs, Discworld fantasy and Ourworld fact. I probably should have read the book twice, all the odd chapters then all the even ones. As it was, I found my attention whipsawed between the two. The total was distractingly less than the sum of the parts.
It's clever, amusing and informative. The back-and-forth style just didn't work for me, though.