I cannot find the guide to get started with using HarfBuzz in my own app.
I looked here:
http://freedesktop.org/wiki/Software/HarfBuzz/
This seems to be the official page of HarfBuzz.
I looked over Download, Building and Development sections. But I must be overlooking something.
I've managed to find a post to the harfbuzz mailing list with some (non working) example code here; however, it's from 2008.
It also looks like the source code itself contains some documentation, although I think it needs to be generated via the makefile.