The first three volumes (the oldest) are held together and are outstanding (and in my opinion a little better than the last 2). Néanmmoins, when we started, we want to go further; we want to know why. In the latter two are also away a little of the basic idea of the Foundations (but who cares when it's well written, well translated). And in those two there is also full of references to his other books (the end of Eternity, Robots of Dawn and other robots, etc ...).
The sad thing is when you close the last page of the fifth book, and it must be said that it's over.
A must read !!