You really want to change everyone back. However right now, if you were you'd probaly have to explain a lot of what happened, which could lead to panic, chaos, or the police may take the books from you thinking them to dangerous.
You can't change them back now, but you decide you'll find a way to later when this is either over, or you have some means making them forget what had happened.
For now, you just head back to your apartment.
You arrive home, walk inside, and drop down on the couch, which in the process destroys the fort you had worked so hard on as a kid. You closed your eyes and thought about what had happened today.
You had encounted a thief who was using the Book of Beast to rob a bank. You're not sure if he found the book on his own, or if someone else had given to him. He was using it for his own selfish means, and had to be stopped. Still, you had killed him.
You wonder what spell he was desperately trying to cast before you used the decay spell. Would he have gone as far to kill you too? At the moment, in desperation you assumed he would, but now you wonder if you did the right thing.
You push these thoughts out of your mind as best you can and look at the two books you have currently. Two new spells had appeared in both books when you came into possession of the second. You could probaly assume the same would happen to the others.
The other books.
The Book of Body.
The Book of Myth.
The Book of Transmutation.
The Book of Mind.
The Book of Arcane
You had recovered two of the books, but the other five were still at large. The first two make have been easy to obtain but the odds were low it would be the same for the others.
Also, you would have to balance your own life while chasing after these magical tomes. It was the weekened which made it easy to investigate the books now, but come Monday you'd have to go to work. The odds of your supervisor letting you take the day off to 'search for magical books from another world' were low. You could easily imagine how that conversation would turn out.
At least for now you can rest, and both books and work can wait.