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!
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.