A hymn is "A song of praise or thanksgiving." Praising whom? Giving thanks to whom? Why not call it an Atheist Songbook? The word "hymn" is a little confusing. Sounds like you're looking for uplifting, positive songs. A title like Songs To Lift The spirits (with a small "s' is more like what you're talking about. That in itself is a valid reason for gathering songs together. I think you need to explain why you want to do a book, (or is this just an interesting idea?) and what you'd do with it after you had it?

Jerry