Joe, why not contact them to clarify? Particularly a message in writing can be taken in a very different mood and tone than written, all the more so when we are possibly self-conscious about something like memory, if I may be so bold as to suggest so. It sounds like the event is planned by a group, so it's very possible that a left hand didn't pay full proofreading/content attention to what a right hand was doing, and it may be appropriate to bring that kindly, in your inimitable way, to their attention. "I got this message that doesn't really fit with the invitation I received and accepted--I'm well known to sing with lyrics at hand including lyrics in a book, and yet this policy was sent to me after I accepted the invitation and paid up. So I'm feeing like maybe you're wanting to un-invite me but were uncomfortable saying so." Then be quiet and wait for them either to reassure you mightily or squirm in silence while they find what to say. Or put their foot in it even deeper. Whether they meant to be or not, organizers were unfair and hurtful. You will know from their response either that a refund is appropriate or that you'll be accepted and enjoyed for the performer you are.