Hmmm this is a tough one!

On one hand you can upset her by not finishing the book by saying its just not your cup of tea. You could tell her that you don't think that you're the best person to give her feedback. Of course, she's probably going to get upset.

On the other hand, you could read the book and give her your honest opinion. This likely won't go well, either.

But, there's a third option...you could read the book and do the half-truth thing. Find a few things that you like about the book and comment about those things. If she inquires further, be vague and give general positive feedback. I'm a pretty straight shooter and prefer the honest truth, but since she is so incredibly sensitive about any sort of negative feedback, this third option may be the best way to go if you want to save the friendship. (Again, I'm not for lying, but she's not really open to taking honest feedback.)