A writer who helps others write books or other material is called a ghostwriter. Just google the term “ghostwriter” and any number of ghostwriters for hire will be listed.
But do your research before hiring someone. As is unfortunately the case in just about every area of our lives these days, there are scam artists out there. Ask to see referrals from people they have previously ghostwritten for. Read books you have confirmed they have ghostwritten. There are a lot of good, reputable ghostwriters out there but just be careful who you pick and research the person before hiring them.
There are websites on “how to hire a ghostwriter”. Google that term also for additional guidance on how to go about it. Good luck!