It's not really hidden, it just got shoved into a hole like LUE. It
used
to be labelled as a social board which is one of the default board categories but now it's under private boards.
https://gamefaqs.gamespot.com/boards/browse/20-community-boards-private
I think the old "Special Social Boards" (YSB, ZSB, etc.) are probably more hidden despite not having any entry requirements I know of. Not sure they even have a category/are listed, but not 100% sure on that.
May you find your book in this place.
Formerly known as xp1337.