The title pretty much describes whole idea. Though Patchouli does allow to lock individual pages behind advancements, completely preventing them from appearing, it might indeed be very difficult to properly convey to player that some entries might be expanded later on. When they suddenly get expanded, Patchouli itself does not provide any clues that it happened. Though there are other ways to notify player about this event (and they should probably be implemented by book creator in any case), it would be pretty neat if entries with locked pages were simply marked as unread again once some more or all of these pages are unlocked, as additional straightforward indication that there is something new in them.

While on the topic, it would also be useful if Patchouli did allow to create inverted advancement requirements, thus showing entries and pages marked with such only until specified advancement is unlocked, concealing them afterwards. I primarily had this idea as another possible way of creating convenient indication for hidden pages. It can allow placing in entries some pages that would hint the player that entry can be expanded and possibly containing clues on how it can be done, later automatically concealing them when they are redundant and not useful anymore. This might be a very specific feature, but I believe it shouldn't be any difficult to implement it.

I would like to see this, too. I am working on a mod with lots of unlockable content, and not every page needs a complete new entry. Setting entries to unread when a page is unlocked would help a ton.