Yep--there are lots of tune book apps, and some of those will aggregate tunes into printed or PDF books with a TOC and index. I was just thinking Deepdwn might provide more flexibility, but as you say, it's really a Markdown editor at heart. It would be great for writing text with musical examples, which I'm guessing is closer the the original purpose for including ABC.
ABC is similar to Markdown, in that there are a proliferation of extensions and roughly a jillion apps that generate everything from butt-ugly screen rendering to LaTex or MusicXML output suitable for professional printing; hence, I'm not surprised my wonky formatting didn't work. Returning to standard ABC fixed it.
My other reason for asking is that I've been working with apps like Ulysses and Scrivener to convert my mind maps into actual documents via OPML export-import (Ulysses works especially well with MindNode, although I prefer SimpleMind). For a writing-phobic geek like me, the ability to break text down into manageable bits is a game changer, and text cards are a big part of that. Well integrated outlining comes close, which is why I decided to try Deepdwn--I need to get comfortable with outline folding, as this accomplishes much the same thing.
The drawback to Ulysses is that it uses wonky Markdown, although the integrated environment and tools make up for that if one is willing to stay there. One can hope that they will allow editing in more standard Markdown in future versions--there is a Markdown option for external files, but even that has nonstandard behavior around paragraph breaks. If I can get comfortable working with Deepdwn, that will allow greater portability of my files between it and other Markdown based apps.
Thanks,
Ken Tryon