Writers on their own, with a book in hand, may need an agent or representative, because agents are closer to the publishers. Agents are also good because they save the writer having to concentrate on marketing work as well as writing it.
One can go it alone in the grand scheme of choices a writer has in getting their work published. Send an email. Try and meet the publisher, even if out of town. This is the hard work of marketing one’s work alone. Some go out to the workforce and work as a writer, full-time. They are in the job as it was.