Another, and I think gentler introduction to ABC is Steve Mansfield's Tutorial.

I'm uncertain as to the status of the abc+ project but would be more inclined to look at the abc 2.0 draft as I think that is the direction abc will take. Most of it is the same but I think it addresses for example problems with the 1.7 draft which I believe was mostly based around abcm2ps (part of the + project). One problem that is well known is that abc2win (pretty well oblolete but a lot of abc's created with it exist) used the ! symbol for a line break and abcm2ps used it for defining symbols on a score. This is documented in section 10.2 of abc 2.0

Jon