Hi Jen,I don't know anything about wire harps. (Why am I answering this thread, I hear you say? :-) ) but firstly, I think you should discuss this option with a harpmaker because you might end up putting a lot of strain on the harp's structure by using piano wire. Don't forget that pianos have strong metal frames, and are built into a box structure, and autoharps have a solid piece of wood behind the strings, not just a frame (soundboard, neck & pillar) like a harp. The harplist people might have some suggestions, or cautionary tales. (You have used the harplist before, haven't you? If not let me know & I'll give you the details for joining.
Secondly, about the book: I don't know if this book is the best wire harp book, but I know that Ann Heyman is recognised as on of the best wire harp players. I think her book is called the Harp Key, which is the same as the name of one of her albums of Scottish music.
I'm fairly certain that the Sylvia Woods Harp Center in California still stocks it. If you need me to find their contact details plese let me know, but I know that they have a website, and you can order books from there, so you can do an internet search for that.
Again, the harplisters will have suggestions, and opinions, on good wire harp books.
Happy harping, Helen