If I'm not mistaken, there are 3 secret tomes, one for each character. But unfortunately you can only read them if you start a new game after completing the last dungeon. I'll not spoil you their use, but don't expect anything great, though. Sorry :p
-Arche's book is located at her house, next to her bed.
-Sana's is at Barness, inside the INN.
-The last magic tome (probably Stella's, not sure yet) is at the Hot springs city (was it Kurat?), inside the mayor's house. (I think you need Unison to reach this one)
Anyway, I also thought you could read them after reaching X lvl, but unfortunately no D: