I have read the original question but have not had the time to read all the responses so forgive me if I am 're-inventing the wheel'.
I think it is better to sing than not to so, if you really cannot remember the song, then use the book. BUT, if the words are coming off the page in through your eyes and out of your mouth then they are not going through your heart! Also, while you are looking at the words your head and neck are not at a good angle for singing.
I find it fairly easy to learn songs so I suppose I'm not a good one to give advice but this is my technique: Write the song out yourself in longhand with the proper verse structure, preferably on a single A4 page. If it doesn't fit use two pages side by side so you can see the whole thing. Put it somewhere you can go and look at it easily (masking tape on the kitchen/garage/workshop/office cupboard door is good!) Then sing it while you go about your work/chores/?. Don't worry about minor mistakes but go and check if you get stuck. Learn it one verse at a time and don't go to the next verse until you've got the first right. Sing the verse(s) you know then the one you are learning and gradually build up the song. And if you inadvertently change a word here and there, as long as you don't destroy the sense/meaning, that's the folk process! Then when you take it out in public, shut your eyes, pretend there's no-one there and let it come from your heart.