The term ghostwriter is so broad.
Imagine you meet a person at an event and you ask,”what do you do for living?”
“I am a ghostwriter.”
“Cool. So, what do you ghostwrite about?”
“Oh, you know, stuff people want written.” with a non-nonchalant shrug of the shoulder.
You nod and move on to other topics with more interesting interaction, but you wanted to linger on that topic. You probably wanted to learn more about what types of ghostwriters are there: Blog ghostwriter or a book ghostwriter? If he is a book ghostwriter, then does he handle certain genres? Sci fi or erotica, maybe?
If it is non-fiction, is it business, self-help, or a memoir? As these are most common book ghostwriting genres.
Then you can continue down the micro-niches: Therapeutic memoir ghostwriter: one who has experience with helping authors guide the writing process so that they can gain closure and heal from trauma.
Another one: Christian/spiritual memoir ghostwriter: one who helps the author find the theme of their memoir to show what led to their spiritual awakening. It is a beautiful work of art to witness someone’s quantum transformation.
This list could go on and on…