a story set on a colonized extrasolar planet;
a second story set on the same planet;
a story set on a second planet;
a story about the ultimate outcome of extrasolar colonization;
a story about the beginning of extrasolar colonization.
The first two stories to be written are amalgamated as Book Three with the very earliest story incorporated as "Cycle One" between "Prologue" and "Cycle Two." Further, there is progress even within this single Book since Cycle One describes microscopic aquatic life in a single pool whereas Cycle Two describes the voyage of a wheeled, wooden "spaceship" to a second pool.
What Blish does not write is a TV-style series with many interchangeable episodes that could be read in any order.