At Google Books:
The Standard Course of Lessons and Exercises in the Tonic Sol-Fa Method of Teaching Music, by John Curwen, 1872. (read online or download)
They have some other old publications on sol-fa notation.

After digging into the system, though, I quickly came to some conclusions:

1) Many parts of the notation don't lend themselves to representation either via plain text files or even HTML (which most people don't know how to code anyway). So it's a very limited choice for exchanging tunes here. ABC is far easier to learn, understand and enter tunes with, since it's so analogous to standard music notation. (Yeah, I know there are lots of musicians who have never learned to read music, but get with the program!)

2) The frequency of temporary modulations by key changes rather than accidentals is far more confusing than the use of accidentals in standard notation, but the only accidentals I could find advocated were fe (IV#) and ta (VIIb). Extension of the "a/e" system seems to be a modern innovation, and from what I've seen, different folks use different syllables.

3) I couldn't figure out how they notate music in minor keys, let alone modes. Did the tonic remain a static "do", with the invervals between degrees changing accordingly, or did the scale shift down to start with "la", preserving the normal interval relationships between each syllable pair?

Bear in mind, this system was developed back in the days when people wrote things down with pen and paper, and music was far more diatonic and predictable. Some enterprising souls have surely revised the notation to be more computer friendly, but I don't know if a standardized version has won wide acceptance.