I purchased the Return to the Planet Apes many years ago and love the show. It is too bad that it only aired for 13 episodes. The last episode left you hanging as both Bill and Cornelius were on their way to bring the book back to the Senate.
There are few things that are puzzling with the show:
1. Colonel Ron Brent's character and how he was used. Brent shows up in Trail to the Unknown and for the next two episodes and then you do not see him. I would think he would have a vital role in series, but the writers focus on Judy, Jeff and Bill.
2. Underdwellers and Krador role as different as they were also in about three episodes, i.e. The Unearthly Prophecy, River of Flames and Invasion of the underdwellers. I would think the underdwellers would play more a vital role in the series, especially tying to relate it to last movie were the mutants took on the apes.
3. There was not enough plot with Jeff/Bill exploring the planet, i.e. former airbases and/or rocket sites. You would think they would look for ways to leave the planet.
4. We only ever see Ape City and no other ape settlement. Yet the show makes reference to Shakespeare as Apeshakespeare or the Ape Father (God Father), etc. So there were other Ape settlements.
If i was a writer then i could see them having the book presented on the Zoo to the Senate and both Bill and Cornelius being arrested for herasy. I could then see Jeff and Judy helping Cornelius/Bill escape and plot for S2 building around uniting the planet and/or Bill/Jeff/Judy escaping the planet!