The SCEC group is quite amazing in their dedication, promotion, and passion. One of the great things about buying their trees is that you know the varieties will grow down this end of the country! Koanga does similar up north.
From what I can tell, heritage & heirloom are pretty much interchangeable although I have heard someone explain a subtle difference (which I've forgotten). Old varieties still growing, anyway, lol.
I've not heard of that book, so I'll have to look it up. :)