Here are pictures of the descriptions of the other books.
Pure: Pink Hair.
Moon: Blue Rabbit Ears.
Summoner: 2nd Pink hair.
Chronos: Black dress Reaper.
Red Book: Makes Pure form's running bombs deal fire damage. Also changes Moon form's sliding to be fire based like in the first game. Fire resistance +1.
Light Blue Book: Ice resistance +1. Gives blue shots to Summoner form's normal attack. Also improves breathing ability (Low priority)(?).
Green Book: Mirage ability (Overrides guard skill, not for when the main character is in nude state) When on the ground and not moving, all small projectiles will be evaded. Changes all of male's transformed form's normal attack to become like their down + Z one. Also changes moon's sliding attack to be more effective toward human type enemies(?).
Yellow Book: (Overrides guard skill) For every form except Chronos, create a barrier around you when you press down + X. (This is the same book as the one in the first game).
Dark Blue Book: Protects against confusion. (Overrides guard skill) For all forms except when the main character is in nude state, when on the ground and not moving, press X to shoot orbs in a small radius around you.
A lot of these abilities are overridden by equipment or skills provided by equipped summons.
Only seams to work with female MC form so... meh
Is there is only five Subquest?
I only found 5 total.