Christine Boylan & D.J. Milky 
Princess Ai: The Prism of Midnight Dawn, Volume 2 [PDF ebook] 

Ai fights to escape the clutches of her mysterious and malevolent captors, not knowing whether Kent, left behind on the Other Side, is even still alive. A frantic rescue mission commences, and in the end, even Ai’s magical voice may not be enough to protect her from the trials of the Black Forest. Dark secrets are revealed, and Ai must use all her strength and courage to face off against the new threat to Ai-Land. But will she ever see Kent again…?