So, I have idea on how to do this:
the shapeshifter, while the resident wizard is occupied and getting himself deeper into trouble, performs a summoning ritual before leaving with the book. They summon a sarcophagus--precious metals, gems, signs of warding, etc and so forth--and leave it buried in a place where it'll be found soon. Say, in a housing development, where a basement is due to be dug. Giant archeological find, etc and so forth... and whatever is in that case is enthralling the mortals around it. If they manage to get it open... well, nice knowing ya.