It is possible to both have some metrics and not have them be the only way you determine if an employee is doing a good job. Because some things can't be measured, and some can.

There are no objective criteria as to what is "good" writing vs "bad" writing.

> If it is an author directly selling books then you measure by books sold.

This is a fairly lousy metric. It depends enormously on the marketing campaign and the ability of salesmen to sell it.

For example, I read an article about the author of the "Slow Horses" book. It languished for years selling at a rate that was indistinguishable from zero. Then some journalist read it, wrote a glowing review of it, and it took off. Now it's a best seller, with sequels, and a miniseries.

Good writing is writing that allows the publishing house to achieve their end goal and bad writing is that which doesn't. The end goal is the same as for other businesses to make money. If you don't sale books you are a bad writer for their purposes.