Key Techniques for Harp 
by Karen Marshalsay
(Previously advertised under the working title The Wire 100)
Buy the book (UK only) or PDF.
This is a compendium of fingernail techniques, with 100 exercises, studies and arrangements to encourage players and students to learn and use the gestures from the Robert ap Huw manuscript, along with a few other useful ones.
This book will also be of interest to anyone who is intrigued with the music of the Robert ap Huw manuscript, as each of the gestures on p35 of the manuscript are explained and described. These gestures, or fingering movements, are of course essential to play the pieces from the ms, but are also very useful to players of any harp, especially as ways to play decorations and ornamentation in both early and traditional music. They work really well when playing Scottish pipe tunes on the gut-strung harp to clean up the sound and give crisp articulation to the tunes.
A full contents listing is given below.
Recordings of all the pieces in the book are available here.
An alphabetical listing of the fingering gestures in the book along with some errata are noted in this pdf.
An example of how these techniques can be used in arranging Scottish pipe tunes is available here - an arrangement by Karen of Nellie's Strathspey. The easier version uses the half scratch and introduces the idea of placing the thumb onto a string to both clean up the sound and act as an anchor for the hand while playing '3 2' repeated notes. The more advanced version uses bee's plaits, thumb chokes, beats, breaks, half scratches and forked chokes. You can also hear Karen playing each version quite slowly.
Short video demonstration of one of the techniques - 'the beat'