...I remember them being very good ! I was rather disappointed when I saw...episodes and they weren't as good.
Similar situation here. I saw the animated series during its original NBC Saturday morning run and I bought Foster's adaptations as they were released. The cartoon was dropped come Fall of 1975, so from that point all I had were the novelizations and the hand drawn reproductions of cels in Trimble's Concordance.
After reading them multiple times, I started to remember them with ADF's embellishments, Kirk suffering 3rd degree burns upon his hands in "Beyond the Farthest Star" and the extra corridors and chambers explored upon the pod ship. Foster turned M'Ress from a "gimmick" alien into a realized character, going so far as to having her mauled by Kzinti during a flashback when she recalls her past assignments.
Then the series was rerun upon Nickelodeon in the mid 80s and the episodes came across like Readers' Digest synopses of longer stories.
And as already stated, the original 3 episodes per volume resulted in Foster dialing back to just 1 episode per the later books supplemented with original material. Ever wonder what was happening on the Enterprise when Spock, Uhura and Sulu encounter the Kzinti with the Slaver stasis box? Foster has M'ress along with two other female Caitians go feral and try hijacking the ship! Let's just say Scotty's solution to the the crisis was "unique".