I have several clients who have self-published and put their book on Amazon. You can also build a dedicated page on your website for the book, and allow people to comment, like, etc. I've seen people post small snippets of their book on social media, and then encourage people to visit the website to learn more about/buy/download the book. Still others write a monthly blog post featuring a chapter of the book each month. Don't forget the face-to-face as well! If you do presentations, your book is a great marketing tool. And yes, book signings are still fantastic! I've seen some great successes with the above. Good luck!