My suggestion for an absolute beginner to get initially started is
http://www.eugenecormier.com/pdfs/lilypond-guide.pdf. A few versions
behind by now, but I don't think the basics covered in the guide have
changed between 2006 and now. At only 18 pages including front cover,
TOC, and appendices, it's less forbidding than the ~180 pages of the
Learning Manual/