Now available in .PDF from the Mises Institute, is F.A. Harper’s Why Wages Rise.
It is a short read – 122 pocket-sized pages. I picked up a 1957 paperback printing for $2 at the Mises Institute this summer, from their used book sale. It’s a steal – copies on Amazon.com are going for upwards of $50. In any event, I’d recommend taking a look, you can finish it in an afternoon, easily.