I would second what one poster above said i.e. it's okay in some sessions but frowned on in others - try to find out in advance which type you're going to and take your cue from that. I do think if you're going to sing in public you should work at the song, even if you need to use your book to help you, until you only need to glance at the lyrics, and not have your eyes glued to the book while singing. I always have a songbook with me when I do pub gigs, so I can do requests. I used to be able to retain at any given point well over 100 songs in my repertoire - enough to do a 4 hour stint without repeating a song. Now I find my memory doesn't hold as much, or maybe I just have accumulated too much information over the years and I'm running out of space! So the "pub songs book" comes in handy.