book bought it, I'd be a happy author. And I am. Those downloads
cost me no more than glances at the cover in a bookstore, and the
sales are healthy.

We also like to think of physical books as being inherently
*countable* in a way that digital books aren't (an irony, since
computers are damned good at counting things!). This is
important, because writers get paid on the basis of the number of
copies of their books that sell, so having a good count makes a
difference. And indeed, my royalty statements contain precise
numbers for copies printed, shipped, returned and sold.

But that's a false precision. When the printer does a run of a
book, it always runs a few extra at the start and finish of the
run to make sure that the setup is right and to account for the
occasional rip, drop, or spill. The actual total number of books
printed is approximately the number of books ordered, but never
exactly -- if you've ever ordered 500 wedding invitations,
chances are you received 500-and-a-few back from the printer and
that's why.

And the numbers just get fuzzier from there. Copies are stolen.
Copies are dropped. Shipping people get the count wrong. Some
copies end up in the wrong box and go to a bookstore that didn't
order them and isn't invoiced for them and end up on a sale table
or in the trash. Some copies are returned as damaged. Some are
returned as unsold. Some come back to the store the next morning
accompanied by a whack of buyer's remorse. Some go to the place
where the spare sock in the dryer ends up.