Apparently the physics works out that if it had, and it *was* a primordial black hole, which is really small, it would take a crazy number of years for it to eat up enough of the earth for us to know and care. Like, I forget how many 0s in the number of years but it was more than a dozen.

The book is titled, naturally, Singularity, by Bill DeSmedt. It's a decent free (or pay-what-you-like) podcast, or printed book.