USA Writer are eligible to write anything?

I began my writing career as a fiction writer a few years ago. Now I desire to bring diversity to my writing as I want to write in a different genre. I want to build connections with experienced and outstanding book-writing companies in the USA. Working with a reputable book writing company will benefit me in several ways. For instance, they have vast networks of clients and industry contacts that will help me connect with potential clients and build my professional network. Moreover, I will get the opportunity to work on a variety of stream of projects that will boost my knowledge and experience. So, if anyone knows the best leading book company, please share details.