I wasn't suggesting you immediately (or ever) drop the book. But if somebody has been coming along for years and hasn't memorized anything from it, there's a problem that needs addressing.

I think you sometimes do street performances. That's a nightmare if you have to keep sheet music dry and in place. You don't need many tunes for that, and it makes sense to do the ones that the most people have memorized. If there are just one or two people who have to stop when their page flutters over in the wind, that's okay, but it looks awful if everybody's like that for every tune.

Ideally books like this would have ink that fades, so that memory has to take over gradually. The ink used for British Rail senior citizen annual discount cards goes invisible in six months so it's a solved problem.

Nigel has his tunes in a zillion little A5 booklets. Very handy and flexible, but to start a set you need a master index so they can call out a booklet name and page number. (Suggestion: if you go for multi-volume collections like that, don't do what Nigel did and code them by colour. The group now has a couple of colourblind players who can't tell the purple and green books apart. Pictures of animals like the O'Reilly computing book series would have worked better).