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 two small packets I use when working with composers. I have a lecture presentation I created many years ago when I was first asked to give a presentation to a group of student composers on how to write for the harp. To accompany this presentation I created my "Harp 101" packet. 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. My second packet is a short one which goes in-depth on how to write pedals into a composition. Pedals are a hot topic in the harp world, so this packet is only for those who want to dive deep into writing for the harp.
Writing for the Harp: