Here's why:
* It's a descriptive phrase, not a title. "New York Times best-seller" is a common phrase used to describe a book, not the title of a book itself.
* It's not a work of art. While titles of books, plays, movies, and other creative works are italicized, this phrase is a descriptor, not a piece of creative work.
Example:
* Correct: "The novel, a New York Times best-seller, was a critical and commercial success."
* Incorrect: "The novel, a *New York Times best-seller*, was a critical and commercial success."
Exceptions:
* If the phrase is part of a longer, complete title: For example, "The New York Times Best-Seller List" would be italicized as a title.
Always refer to a style guide (such as the Chicago Manual of Style) if you're unsure about formatting details.