Writing a book does nothing to prepare an author for the necessary work of marketing that book. One of the difficulties with book marketing is the vast number of tactics that can be used along with the opportunity to spend gobs of money while achieving nothing in return.
The Writers and Authors Resource Center has free resources to help you sort out the marketing situation. Book Marketing Options is a free ebook that lists resources you can deploy while the Sampler: Fundamentals of Book Marketing will give you insights into the text-based course.
You can check them out and download a copy of each at https://www.writersarc.com/book-marketing