FreddyHeadey, there's a second book out, "Rise Again" that Joe Offer from here put a lot of work into.

Every place and every singing session has its own norms. AFAIK, in the US and the UK, it's polite to let the song leader do the verses, and join in only on the choruses. It's not normal to sing everything, and it drives me nuts when people do. If it's normal in your session, that's fine, but it's not the way it's done everywhere. And let me say that if you hum along in what you believe is a quiet manner, it may not be so quiet.