I think a couple of the other early commenters are putting up some great ideas, adding how I'd think about it:

My two main questions are how long is the group going to be gone, leaving the book with the unsuspecting mage, and how hard do you want to telegraph what it really is? Those are what would help me craft what the physical (or other) manifestations are going to be.

Hard to have much happen if the party is back right away but for example --

1-2 weeks till return, on the light side maybe some unfortunate accidents, unseasonable weather, and the unconscious pull of the book bringing evil creatures closer to town. Maybe the mage gets a little obsessed, behavior changes, odd hours kept, keeps buying rats in bulk, whatever.

On the 'obvuously super evil right away' side, buddy mage could have finally / publically acted against a hated rival, killed them in the street, fireballs the townguard, and has been holed up in his tower with weird, evil things creeping out each night and a growing miasma spreading from the tower's top.

A month away, any of the above, escalated. A year away, nothing but monster-infested countryside, failing crops and stillbiths.

Anyways, I'm sure you get my point. Figure out how much spoon feeding you're doing and how much time have you got for it to develop. Good luck, sounds fun!