I can't read music or tablature, and/or whatever the hell it is the RUS has at the end of each tune they include, and learn everything by ear. So what I want from a songbook is words to all verses (because I don't learn just from hearing as fast as I used to), and chords above lyrics (on all verses, as WYSIWYG pleads for!), so once I've heard a song I like I can sit down with a book and play it over and over until I no longer need the book. I don't have anybody to play or sing an unknown melody line for me, and haven't a clue how to get it off the computer.
And tho not quite as easy to read, I'd even go for chords inserted in lyrics line--either in darker print or in parens-- if that would allow for more songs per book.
IF I could read music, I'd certainly want a melody line!