This is the complete trilogy in three softcover volumes.  This is it.  Started in the midst of the Second World War by a 21-year old Asimov, and wrtten over the ten year period that saw the advent of the atom bomb and rocketry.  THE science fiction trilogy, the one that started it all.  Star Wars owes a big debt to this, to name just one.  Here's Nobel Prize-winning economist, Paul Krugman writing on the impact the trilogy had on him as a teenager.