Pebble in the Sky was Isaac Asimov's first ever published Science Fiction novel and it is one of the books in the Empire series which is the precursor to the Foundation series. It includes The Currents of Space and The Stars, Like Dust. This series was written very early on in his writing career and it sometimes shows. Also, over time it was proved that many of the Science Fiction elements in this series were actually incorrect. Things like radiation's effect on humans and such.

As such, the series is not in print anymore. However, if you frequent used book stores you can usually get a set in a couple of years.