The best source of 'cheap' books is the library. The only cost is a little bit of inconvenience. Yet library usage is dramatically down over the last few decades. This suggests to me that there is little demand for cheaper books.

The biggest 'cost' to reading a book for many is not the money spent to acquire the book, it's the several hours of leisure time required to read the book, time that could be spent doing something else.

This suggests to me that the overall book demand curve is fairly inelastic.

That Amazon sees more elasticity is not surprising: they're conducting experiments on part of the market. Price has a big influence on what book people buy and where they buy it; but I hypothesize that it has little effect on whether or not they actually buy a book.