I think the LilyPond documentation (http://lilypond.org/doc/v2.18/Documentation/learning/index) is a pretty good way of learning LilyPond. It's linked to from "menu > LilyPond manual" within Hacklily. Maybe I should be more aggressive about highlighting that...

Making community features is a natural next step which might help with finding examples.

LilyPond has a bit of a steep learning curve, but you can absolutely use it professionally. http://www.mutopiaproject.org/ shows what LilyPond can do. Personally, I find that I can be more productive with LilyPond than with Finale, and that the results look better.