Might be a bug, yes. I visited the statue of Vandart, killed Enthan, defeated Zhitenxa, Xirenze and Yurgimar, and helped Arzodius, yet I never got any of these characters' books. Didn't get Yardeniox either.
Also Axobada's book focused on his origins and that he wore full plate armor to hide his wings, but not much is mentioned about the chaos he made or which dragon possessed him or how he got all power or how he was imprisoned. That's why I asked you.