Tricky sentence construction! The problem phrase in your sentence is who turn out one best seller after another. This phrase is referring to those gifted writers, which is a plural subject. Therefore, the use of turn, the plural form of the verb, is correct.
The problem phrase is called a restrictive appositive. An apposition is a construction that uses two noun phrases next to each other with one phrase adding information about the other. Your sentence tells us that she is a gifted writer; it tells us that she belongs to a group of writers who author repeated best sellers. As a restrictive appositive, the problem phrase is critical to the meaning of the sentence and is not separated from the other phrase with a comma.