"Does the book name an author? Or have their profile? They could be one of these frozen people."

Of course, that would mean a bit of sleuthing of figuring out which frozen person was the author, if the credit didn't come with a handy portrait. Failing that, they could simply explore the castle - perhaps there was an area where livestock were kept, or some sort of stable where the creature lived, whereupon further research notes were stored.

"Failing that, we could explore the castle and ascertain whether they conducted these experiments locally. A creature such as that would need to be stored in a location built for it. Rather distinct, I'd imagine."