All of Karen’s recommendations are excellent, and exactly what I would have recommended.
In addition I like The Complete Idiot’s Guide to Music Theory. It is actually a very good intro to music theory. Sylvia’s book is more harp specific (really lever harp specific), but the Idiot’s Guide covers a lot more ground and gives a better overall understanding of the basics of Music Theory.
Sylvia’s book is enough for you to learn to do what you want to do regarding arranging basic accompaniment patters for the left hand, but I think most musicians can benefit from a more in depth understanding of theory, and the Idiot’s guide gives that, but still stays simple enough that most people can comprehend the entire book without the need of a teacher.