Yes, it is a bug.
The book is white for me when he is in cat form. I didn't have a save in the profile I was using with him in human form, and he didn't change back after I did a couple other things, so I sacrificed Madora to force a change (death penalty for moving a basket). After that, it was marked as owned. His attitude towards the main characters was unchanged, so I assume it is a difference with his form. I'll check that later in the profile I have my main game in.
