Amazon hardly ever gives you numbers that don’t begin with dollar signs. It won’t tell reporters or investors how many Kindles it’s sold, or anything else that offers too much of an x-ray inside its business. “More/fewer Kindle Fires sold than Nexus 7s!” is a tempting trend story, but it’s not really a trend Amazon wants to highlight. It’s a little odd, then, that in its newest quarterly earnings report , Amazon is calling attention to the fact that its sales growth for physical books is flattening out. This isn’t just tossed-off; it’s direct from founder and CEO Jeff Bezos: “our physical book sales experienced the lowest December growth rate in our 17 years as a book seller, up just 5 percent.” This is Amazon’s original business, for..