It may be a phase, but if she really LOVES LOVES LOVES books, then she shouldn't be destroying them.
If it was my kid, I would have made her clean up the mess, throw it in the garbage and tell her that she would not have reading time anymore if she kept ripping her books up,
My kids had books that belonged to me and my husband, not to mention their grandparents when they were little. If my kids tore up even one, that would have been the end of it until they could show some respect for them.
3 is young, but your daughter knows she is destroying something.
Make her put them in the garbage.
Don't leave books around for her to tear up.
She's acting out for some reason. She needs to understand that anything she feels like destroying, toys or anything else, will be off limits to her until she feels like respecting them.
She isn't "reading" books while you aren't watching. She is tearing them up.
Maybe she's trying to tell you that she's not ready for "reading" on her own and her books need to be taken care of by you until she knows the true value of them. She's either bored being left in her room with books or angry that you are leaving her to read them alone.
It seems obvious that leaving her alone with books for the time being is not the best plan.