Hey, sorry for the delay! Apparently itch not only has automoderation for comments, but also doesn't notify you when one's pending approval? Wonder if that's a setting I can toggle somewhere... (Sorry, I'm pretty new to the site.)
Anyway, thank you for playing, and for the feedback! I'll see what I can do about story segmentation going forward (and if there's something I can backport into previous sequences.)
As for secrets, the current design plan is to add one larger Easter Egg (of the type you've already found in Ch. 6) per version, so far placed in the final chapter of each update. So far there are two of these in the game, one in Ch. 3 and the other in Ch. 6. (If you want a hint for the first one... "Your meandering path begins at the dead end.")
There's also the major sidequest, as you mention, which involves completing hidden/optional objectives in each (non-flashback) chapter.
As for smaller secrets and optional interactions, there are a number throughout the game so far. They're mostly intended to be little rewards for investigating suspicious or prominent spots on the map, offering additional tidbits/foreshadowing about the world or characters. The bookshelves you mentioned are probably the most substantial ones so far, but keep an eye out for anything else that might catch your eye.
Regarding the broader nature of secrets in this game, I try to write with the idea that everything strictly necessary to follow the plot should be in the main story, but then there's optional stuff to find if you want to dig deeper or find clues to peek ahead a little.
Anyway, thank you again for the feedback and kind words! I'm always honored when people give this game a chance, and I'll do my best to make it the best that it can be in the versions to come!