Resources
There are many websites dedicated to demystifying writing for the harp. In my experience, the best website I have found is an encyclopedia created by Gunnhildur Einarsdóttir. She goes amazingly in-depth and provides clear text and video presenting the wide array of colors the harp is capable of. If you are considering writing for the harp, I highly recommend her website!
Carlos Salzedo was a major figure in the development of the harp and created many of the notations we use for extended techniques today. His publication “Modern Study of the Harp” was published in 1921 and is now in the common domain. I encourage all composers (and harpists!) to explore this text.
Living harpist-composer Bernard Andres also created multiple new extended techniques which he used in his compositions, and harpist Isabelle Perrin created this document to share his notations.
As for my own writings, I have a small packet I use when working with composers. Each student receives a copy of this packet which covers the basics of writing for the harp, ending with a small listing of extended techniques.