How long must a book be to be a book?

Why are books that should be 60 pages long so often 600 pages long, asks author Sam Harris in a recent article. He did so only in passing while building another argument, but it's a matter worth some consideration.

Harris's answer is that publishers need the books longer, so they can sell them for more. The suggestion is that there is some practicality about book production that makes it economically attractive to have longer books. But what happens to this justification when you're dealing...