Most people on this thread seem to have at least some tolerance for some memory-aiding devices in a singaround--which is in fact the topic here, not paid performance.

I would also say that there's no problem with most types of assistance.

But, a few observations:

"Is it OK to sing from a book?" is the topic.   That seems to me different than just glancing at a cue card if necessary--or even holding a sheet with the words to your song.   I'd have no problem with somebody holding a sheet with words--as long as he or she did not have to read every word off the sheet.

Also, if you sing from a book, you set the bar very high for yourself. The song will have to flow, it's best if it has a chorus--and you will have to make eye contact with your audience often--or at least make it clear you are not just reading out of the book.   All this can happen--but it's rare.

If you don't sing from a book , I find audiences are far more forgiving on all these points. What counts is not perfection, but that you have made an effort to learn your song before coming to the venue.

As I noted, I think it's fine for folkies (amateurs) to hold a sheet with words--(not reading the words off the page).   In fact as a talisman it may well ward off stage fright just to know the words are there. Even better if the sheet is folded up and you don't look at it--and that can also serve the purpose.

UK folkies, and the original poster is in the UK, have it much better than US folkies--- in tbe UK there is no chance of Rise Up Singing ever appearing at a session.   Every other book or aid is just fine--but RUS is just the kiss of death to a good session.

Reason:   it makes it just too easy to never even bother to learn a song--and too many people take advantage of this feature.   And if there is one, there is likely to be more than one--which kills spontaneity stone dead and virtually always turns the session into a secular hymn sing. If people would not use RUS as a hymnal, it could be useful. But there is zero chance of that.

As Sandy Paton put it in an earlier thread on this topic:   "Use a crutch continuously and the leg you are taking the weight off will eventually weaken." RUS is a crutch--a strong one--and far too tempting for some people. It's great to learn some songs--at home. And that's where it should stay.   Unless you are leading a group of children.

Any other book or aid is much less of a problem.