Alas, I haven't read these Asimov books yet but I am obsessive about reading things in order. There is a site called Fantastic Fiction (that is indeed fantastic) that lists all series for a given author in order. Perhaps it can guide you?

Pebble in the Sky is the next book although, I believe it was written 1st.

The Foundation story line had been filled in at various times. The orginal Foundation was a trilogy written in the 50s: Foundation, Foundation and Empire, and Second Foundation. Later Foundation's Edge and Foundation and Earth were added as books number four and five in the 80s. Prelude to Foundation and Forward the Foundaton are prequels written in the late 80s.

Pebble in the Sky is part of the Empire Series that comes before the Foundation series. The first book in the Empire series is The Stars, Like Dust, followed by The Currents of Space, then Pebble in the Sky (although Pebble was written first) and the short story Blind Alley.

The Robot Series started seperately from the Foundation Series but merged with the Robots and Empire. That series consists of a bunch of short stories starting with I, Robot and the The Complete Robot, the the novels The Caves of Steel, The Naked Sun, The Robots of Dawn and then Robots and Empire. These books are first in the Asimov timeline although there were written from the 50 through the 80s.

Like you I have to read books in order and I hope this has not put you in a tail spin. After Robots and Empire is The Stars, Like Dust.