"She has twice as many books as him."
Translation:Lei ha il doppio dei libri di lui.
The word "come" is generally used for comparisons of equality, for instance, "Lei ha tre libri come lui." or "Lei è così carina come lui." They each have the same number of books and are both just as cute - equality.
The example above is making a comparison of inequality. "Lei ha due volte i libri di lui." They do not have the same number of books - inequality. In the case of inequality, our options are "di" or "che" depending on the comparison made.