Lucas ST was always evolving, as it should, obviously over time. But not all versions are adressed in the book, just the relevant one and given its a PT focussed book and the PT and
TCW were his last big thing, that is what this book and this books ST version is about and evolved from.
In the 70s,...
Lucas toyed with
SW as a pulp space fantasy series like Flash Gordon that despite being a Saga he split into parts would feel way more like an ongoing serial following the main cast (to which not always Han Solo was counted even). The "Sequel" developed for movie but turned book "Splinter of the Mind's Eye" is the best example here. But as he actually developed TESB and thought about its potential continuation in a trilogy, his work got way more cyclical and found its narrative structre (ring theory, Campbell, etc.).
In the 80s,...
After TESB, the question became "Should
ROTJ be the end, or is there more after it?" and despite plans for more after it ("There is another", The Emperor as main bad, finding Luke's sister (not Leia), etc.), it was decided to make
ROTJ an end. Hence the changes to its original plot, bringing in the Emperor early, making Leia his sister and such changes.
After that, obviously fans wanted more and with comics, rpg and games expanding the universe slowly, the ghost of an ST ever coming was ever present and based in the old abandoned concepts. Will the Emperor return? Will the dark side forever dominate Luke's destiny, given his Force vision in the Tree-Cave-Temple on Dagobah? Is Boba Fett dead?
In the 90s,...
Still continuing the late 80s trends and rumors about a ST coming one day hopefully, the Legends Expanded Universe grew and explored the post
ROTJ as a Lit version of Sequels working many concepts and ideas into various sources, sometimes even in several instances and variations. The Emperor returned, Luke fell and got redeemed, the next generation faced the same fears, challenges, falls and redemptions. But the announcement of the PT after the Special Edition turned the focus on the formerly off limits past and Clone Wars. The hope for an ST was not forgotten but no longer the focus.
In the 2000s,...
The PT underway or done and
TCW ongoing, it was finally time for an ST. Now or never kinda, and all coming full circle it needed the PT and
TCW as much as the OT as foundation. But technology still was not where Lucas wanted it to be and even his Underworld tv show hit delay after delay. And the more
TCW advanced, the more concepts he seeded in it for what he really was working on with Underworld and the ST. As Underworld and the ST though were pushed further and further back again, their concepts in part also got reworked into
TCW episodes in later seasons maybe. Some at least, that could double as seeding any future potential ST.
In the 2010s,...
Frustration on a high, Lucas creativity would not stop, but a sale to Disney became more and more likely as he still continued to figure out his ST and technology.
The rest is history.