I think some of the best for beginners to theory are not necessarily specifically for harp.
Edly’s Music Theory for Practical People is an excellent introduction to music theory. Particularly for those who don’t play classical and maybe even totally play by ear because although he provides written examples of music he also explains things in a way that you don’t need to read music to understand.
The Complete Idiots Guide to Music Theory is another. It begins with reading music, but if someone already can read well they can skip and go right to the next sections. Goes over scales, the circle of fiths (and fourths), modes, chord structure etc… Despite the title it is an excellent introductory book.
For things specifically for