I agree that a singer can interpret, or present a better performance, from memory than from reading. However, some of us are much better at memorizing than others--its a simple matter to set the rules for your session - if you don't want books , say so in advance.

On the other hand, you may be missing hearing something you may enjoy.
To my regret, I find it very difficult to remember words to songs. Why? I have a medical condition that has affected some of my cognitive abilities. Do I want to have to explain this to session participants? No. I would much rather you make your preferences clear before the session, so I could gracefully decline.