This is another question related to the previous one.
As you may have noticed, Swan claims that (a) is wrong. Again, I believe we have a sentence like (b):
(a) His book is selling itself well.
(b) His book is selling itself.
Here's my take:
"The ____ sells itself" is an idiomatic expression. "His book is selling itself well" is not wrong, but would be a very unusual construction. "His book sells itself well" would be more common.
I also think "The book sells itself" is also fine. But that doesn't mean that the book stands up, offers itself for sale, and takes the money when it's sold. It means that the book is so good that when someone at the store reads the beginning or hears how good it is they want to buy it. There are similar expressions about other objects, like "this car sells itself."
As a matter of fact, the problem with (a) is the adverb "well", IMHO.
Correct me if I am wrong, please.